Are you having a specific problem?
What OS are you using?
There are a few obvious things you can (for XP for example) do such as:
1)turn off system restore.
2)defrag your hard drives.
3)turn of all unnecessary services (per black viper or other)
4)turn off indexing on drives and service.
5) run spybot (free)
6)run a full virus scan (free avg 7.5 or antivir are free and not too bad)
7)ctrl-alt-del and check all processes running - check them against a reliable startup program database (plenty of sites on net)- disable all unnecessary.
8)check for things that are set to run automatically on bootup - disable in software settings or services - if they're not needed running in background all the time. if no option or service to disable, goto start>run> type msconfig and disable from there.
9) run a decent firewall
10) fdisk/format/reinstall OS anyway
