Our printer is saying it has stopped because the photoconductor kit has reached the end of its useful life.

Here is an excerpt I found from the W840 user manual. Looks like you need to change the photoconductor kit. Here is the link for the user manual: images.lexmark.com/publications/pdfs/w840/eng/ug.pdf
To ensure print quality and to avoid damage to the printer, the printer stops operating after the photoconductor kit has reached approximately 60,000 pages (Letter/A4). The message 84 Replace PC Unit is displayed when the printer reaches this point. If the Toner Alarm setting is set to Single or Continuous (default is Off), the printer automatically notifies you before the photoconductor reaches this point and displays the message 84 PC Unit Life Warning. To avoid having the printer stop operating, replace the photoconductor kit when you first receive the 84 PC Unit Life Warning message. Once the photoconductor kit has been replaced, normal printer operation resumes

If the printer has been in use for some time, then this message is correct as parts do have a limited service period and need to be replaced.

84 PC Life Exhausted indicates that the photoconductor unit has reached its functional life. The photoconductor kit will need to be replaced. The printer will not function so long as the Exhausted message remains on the display. The only way to clear the message is to install a new photoconductor. After replacing the photoconductor unit, you need to manually reset the pc page counter as well. Thank you for choosing Lexmark.

The light pattern indicates that the photoconductor kit has reached the end of its useful life. Replace the photoconductor kit then reset the photoconductor page counter. Thanks for choosing Lexmark.

The photoconductor kit is another word for the drum or imaging drum. All three words refer to the same part.

Here is a link from the Dell site for this model.
http://accessories.us.dell.com/sna/PopupProductDetail.aspx?c=us&l=en&cs=dhs&sku=310-5404

Once you replace the drum, the printer has to be manually reset. If this is not done, the printer will continue to give this print out. By resetting it, the counter is set back to zero. The drum is good for around 30k prints.

From the Online Manual for the 1700, to reset the counter:
http://support.dell.com/support/edocs/systems/1700n/en/index.htm#online_documentation

To reset the counter, press and hold the Cancel button until all lights flash in sequence.

NOTICE: Resetting the imaging drum's counter without replacing the imaging drum may damage your printer and void your warranty.

After replacing the photoconductor kit, you will need to reset the photoconductor page count. Here's the link for instructions on how to reset the page count: http://bit.ly/agYtVo . Or simply press and hold the CANCEL button while the printer is on and the front cover open until all lights are flashing. Hope this helps. If you need more assistance, leave a comment and I'll get back to it. Thanks.

Not sure about the part #, but photoconductors and imaging drums are the same thing.

you must reset the error message; find the explanation in the photoconductor kit package

To reset this error, open the front door then press the cancel button for 3 seconds - all lights will cycle, close the front cover. The counter is now reset
You should of had some instructions that came with the new photoconductor kit that shows you how to do this.
Hope this helps.

Here is how to reset the photoconductor kit counter:

1. Make sure the new kit is installed and the printer is turned ON.

2. Open the front panel.

3. Hold the Cancel button in until the front panel LEDs flash.

4. Close the front panel