There's tons of posts on here about how to speed up a slow computer... here's my .02 worth. The usual disclaimers about backing stuff up before you do anything apply here...
1. Remove any un-necessary programs...
This includes removing programs you don't need to free up disk space and any programs that run at startup that you don't really need to free up memory.
2. Setup your page file correctly...
Right click My Computer -> Properties -> Advanced Tab, under Performance click the Settings button. Click the Advanced Tab and under Virtual Memory click the Change button. Select the C drive and make sure System managed size is selected. No matter how big you set this file to, Windows won't use more than what it recommends so this is typically your best option. If you have more than one physical drive, turn the paging file on the C drive off and enable it on another drive. You will have to reboot if you change this.
3. Clean up the junk...
Download
CCleaner.
-Run the cleaner and run the registry cleaner as well. The registry cleaner will prompt you to make a backup before you make changes.
4. Defrag your hard disk...
There are several good (free) defragers available, I prefer Auslogics.
Auslogics - Download Auslogics Disk Defrag
5. Defrag your registry....
Auslogics - Download Auslogics Registry Defrag
Auslogics makes a very good registry defragger as well.
You will need to reboot after this is done.
6. Defrag your page file...
Sysinternals (Now owned by my Microsft) is a great utility for doing this. It takes a few minutes at most the first time you run it, after that it's a matter of seconds to keep it defragged. I have this set to run at every boot on my computers, it only takes 2 seconds.
Download:
PageDefrag
7. Speed up your boot process...
Google "bootvis" and download it. This is a tool that was developed by Microsoft (although they don't support it) that will reorganize the files needed for booting up your PC so it boots faster. I have used this on several computers and more often that not, especially if the computer is older, you will see a noticeable difference in the amount of time it takes to boot up.
8. Make sure you only have one AV program running...
Multiple antivirus programs can slow a computer down.
I run CCleaner on a weekly basis and I defrag my hard drive and registry typically every 1-2 months. Even that's probably more often than I need to.
If you have any questions feel free to post them back here.