Here are some suggestions:
1. Backup all of your personal data.
2. Reformat and reinstall your OS (e.g. Windows XP or OS) to ensure that spywares, trojans, adwares, malwares, graywares, and all X-wares/viruses/worms/malicious codes are removed from your existing machine.
3. Ensure you install Windows Service Pack 2 (before u plugin to internet via dial-up or cable or dsl)
4. Install a good antivirus + antispyware + firewall
5. Plugin your computer to internet
6. Install Microsoft Windows Service Packs (up to the most recent ones)
7. Ensure your Antivirus is having LATEST antivirus definition files (normally daily updated antivirus definition files and anti-spyware definition files). Do a manual antivirus update if needed. Most antivirus sw have enabled auto-update which is the best and recommended setting.
8. Install latest browser versions and ensure its latest service pack is also installed.
9. Ensure that you have only 1-2 local administrator accounts with a complex password (at least around minimum 8 to 10-characters with mixed capital, small, alphanumeric character for a pw combination). At least 2 local accounts in case you forgot the main local admin password. Some rename the local administrator acct for security and logical reasons.
10. Do not create folder shares nor install Peer-to-peer software that auto-share your folders.
11. Do not go to porn websites (they normall attach trojans and spywares that eat up most of your machine resource)
12. If you install new software, if you can avoid not to have it configured to start during computer restarts, do it.
13. Set unneeded applications/OS "services" to disabled or manual if not frequently used.
14. Maintain at least around 2-4 Gb of system drive space available on your computer.
That's it, enjoy a hassle free and turbo-charged PC.