What particular things are slow? Launching applications? Dragging windows around? Web browsing? Did the slow down come immediately after switching to the new monitor? Have you tried temporarily switched back to the old one? Yes, if you have a really cheap video card then it could be responsible.

Get rid of all of your spyware programs except lavasoft adaware or microsoft antispyware beta. Some of the others ones actually INSTALL spyware (tricky, eh?).

Are you running XP? Right click on the taskbar and choose Task Manager. Click on the performance tab. Look at the "MEM Usage" and tell me how much ram you're using. You should look at this value immediately after bootup, and after you've been using the computer for a while.

Run defrag.

After you report back I'll tell you a few more things to do.