Some things you should do on a regular basis is to make sure your Anti-virus is upto date, and then run a full scan at least once a week.
Do a defrag of your disc once a month, maybe for the first time you do it you might need to do it 2 or 3 times.
Run a scan disc on your disc.
Do a disc clean up before you do a defrag.
If you dont have anti-virus a free one that is good is AVG, also you can download Stinger.exe from McAfee which will do a good job at searching out any viruses. Then I would recomend buying a Virus scanner.
Never open e-mails that you do not know who they are from or any attachments
I hope all of this helps
