MFC9840cdw stays offline in vista

The printer must just be offline. Try these to fix the problem.
Solution 1 On the Computer:
Click Start and then go to Printers and Faxes (XP), Printers(Vista), Devices and Printers(Win 7)
1. Right Click on your printer Icon 2. Click Open or See What's Printing 3. Click the option that says Printer on the upper left corner of the window that pops up. 4. Make sure the only thing that is checked under that menu is Set As Default Printer. 5. If Use Printer Offline or Pause Printing has a check mark. Uncheck it.
Try printing again. I hope this helped.

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___Also look this> HP expanded the battery pack recall on May 19, 2010. Click here to validate your battery. Performing a system recovery removes all software and files saved to the hard drive, reinstalls the original operating system, and then configures the settings for the default programs. Since the merger of Compaq and HP, several different recovery tools and processes Names for these tools include: Recovery Solution, Recovery Manager, F11, Recovery Disc, Driver and Application Recovery Disc, Roxio BackOntrack, Microsoft Backup & Restore. have been provided for recovering notebook PCs to their original operating condition. To help you select the correct recovery process, the table below identifies the different models, the time frame when they were produced and sold, plus the originally configured operating systems. Click on the name of the document in the right column for detailed information. Choose the Correct Recovery Process for Your Notebook Notebook description Recovery procedure information HP and Compaq Notebook PCs and Mini PCs shipped with a Windows 7 operating system. Recover Windows 7 Operating System Using HP Recovery HP and Compaq PCs using the Microsoft recovery feature for Windows 7 instead of an HP solution. Recover Windows 7 Operating System Using Microsoft Windows Backup and Restore Presario, Pavilion, and HP Compaq Business PCs shipped with a Windows Vista operating system. Recover Windows Vista Operating System Using HP Recovery HP and Compaq PCs using the Microsoft recovery feature for Vista instead of an HP solution. Recover Windows Vista Operating System Using Microsoft Windows Backup and Restore HP and Compaq Mini with Windows XP using the HP solution. Recover Windows XP Operating System On Mini Notebook Using HP Recovery HP and Compaq Mini with Windows XP using the Roxio recovery feature instead of the HP solution. Recover Windows XP Operating System Using Roxio BackOn Track HP and Compaq Mini with Mobile Internet Environment (MIE ). Recover Windows MIE Operating System On Mini Notebook Using HP Recovery Presario, Pavilion, and HP Compaq Business PCs manufactured and sold in 2006 or later . Recover Windows XP Operating System Using HP Recovery (2006 or Later)

Presario and Pavilion PCs manufactured from 2001 and 2005 . Recover Windows XP Operating System Using XP Operating System Discs (2001 to 2005)

And

Recover Applications Using Drivers and Applications Recovery Disc (2001 to 2005) Pavilion PCs with Windows Media Center edition of XP manufactured from 2001 to 2005 . Recover Windows XP Media Center Edition Operating System on HP Pavilion PCs Using HP Recovery (2001 to 2005) Compaq Presario PCs supplied with a Compaq Recovery CD and manufactured from 2001 to 2005 . Recover Windows XP Operating System on Compaq Presario PCs Using HP Recovery (2001 to 2005) Omnibook and certain Pavilion PCs manufactured pre-2001 . Recover Windows XP Operating System and Applications on HP Omnibook and Pavilion models Using HP Recovery (Pre 2001)

Click Start and then go to Printers and Faxes (XP), Printers(Vista), Devices and Printers(Win 7)
1. Right Click on your printer Icon 2. Click Open or See What's Printing 3. Click the option that says Printer on the upper left corner of the window that pops up. 4. Make sure the only thing that is checked under that menu is Set As Default Printer. 5. If Use Printer Offline or Pause Printing has a check mark. Uncheck it.
Try printing again. I hope this helped.

the first thing to do here is uninstall all printer drivers as you might have a conflict, then reinstal the vista drivers and start from there

I take it you the Win XP PC is on the same network that your Vista Laptop is on and that some how you must be able to share the printer through your XP PC?
I think you are experiencing the many issue that Vista has to offer.
Maybe think of actually seeing about placing the printer on a Network print server. This would solve your issue. I did this with my Brother Laser printer and it works great. I believe the print servers are coming down in price I am using a TRENDnet TP100 and it works great with my two wired network desktop PCs and the two wireless notbooks that both have Vista on them.
Blessings,
A/V Media Guy

Are you attempting to print a double sided page? If so then on your Vista pcs you should set it in the printing preferences to print double-sided whenever you need to print.
Do not give command for two-sided printing from other programs such as Adobe reader or Microsoft Word etc.

Press the most left button at the printer front panel.

switch the resultion to 600*600 and all is fine.

Are you useing vista print drivers for the vista computers or are they XP drivers?

Here is the answer for Vista support on your printer from Brother:

http://www.brother.com/E-ftp/windowsvista/us.html

Does it print on the Windows XP computer just fine?