Here's just a quick and dirty list of things you can do.
1. Disk Defragment
Defrag your computer regularly to speed up access to data. I have mine set to automatically defrag weekly. Start menu ---> Programs ---> Accessories ---> System Tools ---> Disk Defragmentor
In addition to Disk Defragmentor, there is a nifty free hard disk cleaner called CCleaner that I use at least weekly as well. This is similar to the Disk Defragmentor but also includes a nifty registry cleaner.
CCleaner - Optimization and Cleaning - Free Download
2. Disk Cleanup and removing unused programs.
The disk cleanup tool will help you free up space on your hard disk. It will help removes stuff like temporary internet files and delete files in your recycle bin. Additionally, under More Options, you can remove windows components that you don't use and remove installed programs that you don't need or use. You can also remove programs through the Add/Remove programs utility within the control panel.
Start menu ---> Programs ---> Accessories ---> System Tools ---> Disk Cleanup
Start menu ---> Control Panel ----> Add/Remove Programs
3. Adjust Visual Effects
The normal windows default settings are nice but they can take up some valuable resources such as system memory that you may be short in. You can try adjusting these settings to see if that helps.
Right click on My Computer (or to to Start and right click there in case you do not have that icon) and select properties. Go to the Advanced Tab and under Performance select Settings. Select Adjust for Best Performance.
4. Remove Unneeded Startup Programs
There's also quite a few services and windows services that I usually shut down to help speed up my home computer. That is a little bit more advanced but I've used Black Vipers Website since I had Windows XP. You can find it here.
Black Vipers Website
If you scroll halfway down the page he has a section called Popular Content which lists the windows services by your Windows version.
5. Scan and Remove Viruses, Spyware, and Adware.
There's lots of tools out there that are free to assist in this. I personally use Adaware for my Antivirus and Spybot Search and Destroy for Malware and Spyware. Another highly recommended freeware antivirus is AVG. Another new kid on the block for spyware is Malwarebytes.
Malwarebytes: Malwarebytes : Free anti-malware, anti-virus and spyware removal download
AdAware: Ad-Aware Free - Download Internet Security Software with Antivirus & Spyware Removal - Lavasoft
AVG: AVG - Antivirus and Internet Security | Virus Protection
Spybot The home of Spybot-S&D!
Another issue may be hardware. Can you list your system specs and operating system?
If you don't know how to do this and you have a windows operating system, you can right click on My Computer (or to to Start and right click there in case you do not have that icon) and select properties. Under the General Tab, just write out the information under System and Computer.