First off, if you dont have any... go to:
www.download.com
or check the Stickies.
Search for a free ANTIVIRUS, ANTISPYWARE, REGISTRY CLEANER, and possibly a FIREWALL.
First, run Disk Clean Up.
Start>Programs>Accessories>System Tools
Second, open up your browser and clear the HISTORY, COOKIES, and CACHE.
Third, if you want, open up your "My Computer." Right click on your C: drive and click properties. Go to "TOOLS" and find the "Error-Checking" opetion and click on "check errors." You will probably have to reboot.
Then, install the programs you choose if you haven't had any or done already.
Let each one of them run. Preferably the Antivirus, Antispyware, then the Registry Cleaner last. Let if fix the things it finds.
Then, run Disk Defragmentor.
Start>Programs>Accessories>System Tools.
Lastly, if you want, install your firewall if you choose to download one and protect yourself from future threats. Then you can go into Start>Run>"msconfig" w/o quotes.
Click on the "Start Up" tab. Expand the "Command" column.
Uncheck anything that you find you haven't installed and looks suspicious. If you are unsure of it, check with them online.