Slowness in Windows can be caused by a large number of things. Malware, hardware issues, fragmentation, etc. Let's tackle them one-by-one:
Have you tried running CheckDisk from Windows? CheckDisk will check your hard drive for errors and (via certain flags) will try to fix them. Click on the Start menu, choose "Run...", and enter:
chkdsk /F /R
Hit ENTER. It'll tell you that the hard drive is currently in use, and will ask you if you want to run the command at the next boot, and you say yes. Then reboot.
The scan takes a little while. If it doesn't fix the problem(s), open your hard drive with GParted in an Ubuntu LiveCD (GParted is already installed in the LiveCD) and see if it gives you any errors.
Have you removed all malware? I'd recommend starting with Panda Antivirus or Microsoft Security Essentials.
Have you defragged your hard drive lately?
You might download CCleaner to help clean out crap / temporary files and clean up the registry (by the way, cleaning the registry doesn't always help; lots of people are on the fence about its usefulness).
msconfig to turn off unwanted startup items and services (CCleaner can do this, too).
This list isn't comprehensive. Anyone else got any good ideas? If none of these things work and Windows is still slow, then you might just have to purchase a new hard drive.