As usual, haven't been posting when and what I think should go here...
However, due to a new uptick in problems and questions about basic computer protection against viruses, spyware and general bad guys, here are the bare minimums:
You should always have a firewall. (If you don't want to install a third party firewall, at least use Windows' built-in firewall. Do NOT disable it if you have nothing else.) ALWAYS use current, automatically updated antivirus software. ALWAYS use anti-spyware protection. Do not let your PC "go commando." If you do, it will, almost certainly, catch a disease. And it could be quite lethal.
Great, free software to prevent such a nightmare:
AVG's Free Edition (for non-commercial use) You can download it at CNET
safer-networking.org's Spybot Search and Destroy (but don't install the Teatimer option)
I really love SuperAntiSpyware.
If you do not trust these links, go to AVG's website, javacoolsoftware's site , safer-networking.org and superantispyware's site.
These all offer free versions, but some are shareware, meaning these wonderful software writers could use and certainly deserve a donation. But it's not obligatory -- and protecting your computer is.
One last note: If you opt for free versions, ALWAYS do regular manual updates and computer scans on a regular basis. For most users, it is not necessary to do this daily -- but at least once every two to three weeks is a must.
Good luck! - and always use protection!