Just some general tips that might be causing your computer to slow down:
1. Dust - If you haven't cleaned your computer case out sinse you got it, now may be a good time. Dust can build up in the power supply unit (PSU), it'll bog the fan down, causing it to overheat. Although I've never seen one catch fire, it could be a fire hazard. Plus a PSU that overheats doesn't work as well as one that's kept well ventilated. Dust in your computer case can also cause fans to bog down, especially your CPU fan. Your CPU fan is crucial in keeping your CPU from overheating. As your CPU fan decreases, it's temp goes up, once it reaches a certain threshold your CPU will begin underclocking itself to compensate for the increased heat - simply put: a hot CPU will run itself slower than one that's kept cool. Also, just a general rule of thumb - if your computer gets hot, things go wrong; electronics don't respond well to heat, especially your computer.
2. Spyware - Cruising the darker side of the net? Well this can be a bad thing. Adult sites, gambling sites, and just about any other site can cause this. This easily one of the most common computer problems for the average user. The software hides in random locations on your harddrive, embed themselves into your registry, and you'll often know when you have spyware because, as you cruise the web, pop-ups that didn't normally pop up at your regular site start coming up, or if new advertisements are suddenly getting inserted into random webpages that didn't have them before, well, you get the idea.
SpyBot S&D Download Site -
http://www.safer-networking.org/en/download/index.html
Edit: As a side note, consider using a browser other than Internet Explorer. IE's got too many exploits, even with the SP2 and the new pop-up blocker. It's not self-repairing, if things go wrong, they stay wrong until you fix them. Personally, I like FireFox, if nothing else, tabbed browsing owns.
Removing Spyware can be tricky though. I'd recommend Spybot S&D to remove them. There's a few other scanners, and you should always run at least two of them, three or more if you have them. Sometimes some will detect stuff others won't and others will remove things that the other ones can't.
3. Hard Drive Capacity - If you're running out of harddrive space, your system will tend to run slower. Windows needs a certain amount of space on your Hdd for virtual memory (if you run out of RAM, it uses Hdd space to swap data). Keeping your Hdd clean and defragmented can speed up your performance. Some easy things you can to free up some disk space:
Delete your Temporary Internet Files, and Offline Content, see what's listed in your Add/Remove Programs in the Control Panel and see if you have any old software you don't want/use anymore.
Once you've gone through and cleaned house, then you should consider degragmenting your Hdd. It'll probably take a long time, so let it run while you're sleeping or at school/work. But this'll help with your Hdd seek time by keeping data sequencial.
When all is said and done you may or may not see a big improvement. If not, then maybe it's time for an upgrade, consider more RAM, as that's usually one of the easiest and is often cost effective, to improve performance.
Okay, that's my two cents, best of luck.
