Disclaimer: These are just some ideas I'm posting. I'm not being paid for this, I'm not an expert, I'm just writing down some ideas which seem to work for me, so -- before trying any of these ideas -- understand that I am not responsible for any consequences of any of the ideas you try.
If you own a personal computer, chances are that it runs some version of Microsoft® Windows®.
While the ideas used here are mostly applicable to Windows XP®, many of them are useful with other Microsoft® operating systems.
The items below go from most important to least important.
You don't have to do them all at once -- start with the first item to get started with some protection, add the 2nd item for more, etc.
Keep your OS & applications up to date.
When Microsoft® creates a new patch for their operating system, I've heard that virus writers around the world race to understand that patch & the security problem that the patch fixes. Then, they write viruses or other malware (bad software) which uses that security problem to gain access to other computers. The best way you can protect yourself is to keep your operating system and applications (i.e. Microsoft Word®, Excel®, etc.) up-to-date with the latest patches.
One way to do this is to go to Microsoft® Windows® Update every Wednesday or Thursday. Start Internet Explorer, then use the menu pick across the top of the screen to go to Tools > Windows Update. Follow any instructions on download new software, then keep returning to Windows Update until the Express and Custom buttons are on your screen.
Choose as many of the Critical updates as the screen will let you choose, then press Review and Install Updates, then Install Updates.
Often, after you do one update, you'll need to reboot and/or return to Windows Update page to check for others. When no High Priority (=Critical) updates are left, you may wish to consider doing other updates -- but it's important to make sure that you have the critical updates installed.
Install & use AntiVirus software.
Once you are updating on a regular basis, I suggest purchasing Norton or McAfee Antivirus. I've used (and dislike) them both, but feel that the Antivirus protection is important. At least once a week, update your virus protection and scan your system.
Install & use AntiAdWare/AntiSpyWare software.
Click on the Windows Defender Beta 2 link near the bottom center of that page, then click on the orange Download It Here button on the next page.
Beta means test, so Microsoft is offering a test version of their antispyware that's valid for until 31Dec2006.
PC Tools Spyware Doctor (Note: This is free to run, but you may need to buy it to remove spyware from your computer).
Webroot SpySweeper SpySweeper is not free, but you may be able to scan your computer free, just not remove spy/adware with the free version.
Other good antispyware programs exist; these are just a few that I've used & liked.
Install & use AntiRootKit software.
to find the rootkits. Not everything printed by RootkitRevealer is a problem; some things are just normal changes that happened while running the software. I can't tell you how to remove rootkits because I've only had experience with one specific one, and I had to try several things before I was finally able to remove it.