I haven't read all of the replies, so if no one else has mentioned it yet, download startup control panel and disable as much as possible that may be loading at startup and go from there. That is of course assuming that you have removed all adware using Adaware, spyware using spybot, viruses using a good antivirus software, etc...

http://www.mlin.net/StartupCPL.shtml

Some typical steps for troubleshooting sluggishness:

First - Test using by starting up in safemode. Is there still sluggishness?
1. Don't load unecessary apps in memory at startup
2. Clean out temporary files
3. Remove all viruses
4. Remove adware
5. Remove spyware
6. Clean the registry
7. Ghost and then Defrag the harddrive (yikes)
8. Check your virtual memory settings against your hard drive size and free space.

http://www.lavasoftusa.com/software/adaware/
http://www.safer-networking.org/
http://www.mlin.net/StartupCPL.shtml
http://www.registry-cleaner.net/ * I haven't tested this
http://www.pctools.com/registry-mechanic/ *PC Format Mag likes it

**Ghost an image of your harddrive before you do anything (just in case)

One other comment - not to scare you or anything! My computer is just over a year old. It came with a great Western Digital Hard drive. very fast and silent. I never had any problems until about 3 weeks ago I started experiencing 'delays' periodically and a couple of times my computer just stopped responding. I went through the entire gamut to test it - ran thorough hard drive tests which reported no errors, memory tests, verified power settings, switched everything in CMOS and the OS to always on just for testing purposes and also did everything listed above. Even did a complete new install out of frustration. The next day my hard drive failed completely. Just after the 1 year warranty expired of course. I replaced it with a 5 year warrantied Seagate hard drive and reinstalled using my Ghost image and wouldn't you know it - it now runs fast again!