I have a Dell Vostro and would like to lock the laptop and use a monitor and wireless keyboard whilst at home. Can you tell me if and how I can do this?

This is assuming you are using Windows 7. (Since you didn't specify. -_-)
Software lock at same time = No. Hardware lock at same time = Yes.
When you have the keyboard connect and the laptop plugged into the monitor, go to your Windows Mobility Center. Go to "Power Options. Look for "What to do when you close the lid." and tell it to do "Nothing".
Then close your laptop and lock it.
Viola.

I don't know what you mean by "lock the laptop" however in order to use wireless keyboard,mouse and external monitor, follow these steps.
first, plug in the "dongle" for your wireless keyboard and mouse. switch on the mouse and keyboard, (small switch on each one, it may be on the bottom of the mouse and also on the bottom of the keyboard. Your laptop should show an icon in the lower right part of the task bar indicating communications with both mouse and keys. now make sure the mouse and keys are working by opening notepad or some other word processor, type a few lines and move the mouse around, if all works, close that and now....
Plug in your external monitor, and then right click on the laptop screen. if you are using windows 7,click on "screen resolution" when that opens you should see (toward the bottom of window you will see several choices, one might be "connect to a projector" if you do, click on it and choose the option you want, in most cases like this you will want to "clone" or "duplicate" desktop on the external monitor.
Now the external monitor should be working and so try moving the mouse and again open a word processor and type a few lines. If all looks good now you can do a couple of other things
(I am assuming windows 7 with all this) if you have XP the technique is just a little different, but you will need to let me know if that is the case, as i can handle XP as well.
that being said. go to "start", then "control panel" then type in the search box type "power" and a box will open allowing you to change the power settings. What you want to changer here is what happens when you close the top of your laptop. otherwise when you close it, the whole thing may go to sleep. so.... choose "Do Nothing" as a choice when lid is closed. You may want to change some of the others to, but not critical.
Good luck and let me know if this worked for you. I do this all the time and once you do it, the next time you plug in your external monitor it will take right off and you won't have to go through all this again.