I plugged out my PC as it was shutting down. Now it is painfully slow.

Download and burn to CD the HD maker's diagnostics/repair CD to run an extended repair scan:
Hard Drive Diagnostics Tools and Utilities (Storage) - TACKtech Corp.
HD Diagnostic

Follow this with Disk Check

Other troubleshooting steps which might reveal more problems: Computer lags every couple seconds

It's fixed! I ran a checkdsk on startup and it found a file that needed to be repaired and now it's working perfectly! Complete reinstall of the OS avoided. Thanks for the help lads!

I have an XPS720. Installing Windows 7 Ultimate (clean) I found the MS checker showed little problems to encounter there are many as you proceed. Let Windows 7 install naturally. You might be tempted to install Nvidia drivers however be warned they seems to cause more problems than they are worth.
The only things I (had to) additionally install were the Creative driver system (with a few problems as they too were never designed for Win7). Then I encountered 'hangs' and suspected this was due to Microsoft Security Elelments as I use Bitedefender for firewall/anti-virus. I un-installed this and it seems to work quite well now.
Games can be a problem though as I use 'Rail Simulators' and they tend to hang with 'nrvd64' being the problem, again MSSE might have been the cause.
The worst thing is that for no reason whatever the PC might hang. What should have been seemless turned out to be a 'googling' nightmare attempting to sort out the problems. So much for the Dell 'built for the 'future' rubbish they purport to believe!
Michael