This question has come up often enough that I have put together a list of what I use when a person is having problems with Windoze.
There are several things you can do (all the software is free) that will stop nearly 100% of problems on your computer.
1) Hardware firewall with a router and NAT (Network Address Translation)
2) Software firewall - ZoneAlarm
3) Spybot Search & Destroy - new version coming soon
4) Ad-aware
5) Pest Patrol (The free version finds but does not fix problems. )
6) Anti-virus. I am using AVG (
www.grisoft.com). It doesn't drag down your PC like Mcaffee or Norton do.
7) The Messenger service. If you take a look at
www.blackviper.com you can find a list of all the services that do not need to be running (Disabled) and why. Messenger was designed to allow an admin to send you messages. It has been corrupted by others to send ads to your desktop. By blocking all unwanted services you are also increasing your security by minimizing the chances of an infection. Note: This is different from the IM Messenger which I also do not run because it is a risk and intrusive.
8) Firefox (
www.mozilla.org) browser. If you use IE dump it. It is nothing but a haven for pop-ups and pop-unders. Firefox is stable in the 0. 8 version and has a built-in pop-blocker that works very well.
9) Extensions for Firefox - Get Adblocker (Tools | Options | Extensions then click on "Get Extensions"
10) Frequently check for updates for 2 - 6 above and especially Windows. Do this at least weekly.
11) Check out Linux. (My personal favorites are Red Hat Linux and SUSE Linux)
Good luck!