There are a lot of possibilities -- from a temp files and spyware piling up to a need to update your video card driver to defragmentation. Your Windows theme or account could even be corrupted. Or it could be your antivirus package, especially if you're using Norton or McAfee.

Here's my regular regimen for keeping my system running like new:

DO THIS WEEKLY:
1. Manually go to Microsoft Update and make sure Windows is completely updated. In fact, don't just check for priority updates, get even the non-critical updates.
2. Make sure your virus software is updated. (If your virus software isn't set to run in the middle of the night, waking your computer from sleep, for a virus scan, you also may want to do a regular virus scan.) And, every once in a while, go to the web site for your virus software and make sure you have the latest VERSION of the anti-virus software. You may have the latest virus definitions added automatically -- but, if you're still using last year's version of the software, you could still be vulnerable. Even better, use either Avast! Free or Windows Security Essentials -- both free and both much more efficient than Norton or McAfee.
3. Use the excellent free downloads Spybot S&D and Spyware Blaster to clear off and block spyware(update them before you use them and, regularly, check their web sites to make sure you're using the latest versions). You can pick them up from the links on this page:
http://savemybutt.com/downloads.html
4. Run the free, excellent, easy to use tune-up tool Advanced System Care, from IOBit, to tune up your registry and clear off temp and other junk files. This download is available at http://www.iobit.com/advancedwindowscareper.html
5. Run the Defragmenter on all of your hard drives.

And don't forget to back up your files (at least the My Documents folder) regularly! The one piece of hardware on your computer that absolutely WILL eventually fail is your hard drive. If you're lucky, you'll have replaced your computer by then. But if not -- please remember to back up your files regularly.

DO THIS MONTHLY:
1. Check your graphic's card's manufacturer's site, your sound card's manufacturer's site and, if applicable, the RealTek site to make sure your drivers and firmware are up to date.
2. Go to your web browser and clear the cache. In IE, you'll find the option for doing this right under the Tools drop-down. Older versions and other browsers keep them under Tools and in Internet Options. You can leave the cookies -- but do delete the Temporary Internet Files. Accumulate enough of them and they will slow your entire computer system.