I've had ZERO problems on my Compaq Poresario running Vista Home Basic, except that it wont open a few zip files successfully and I have to reboot. Otherwise its amazingly stable and trouble free. I suspect that the "Aero" option on the more upscale Vista versions, plus sidebar programs, are the big memory hogs as they sit there permanently and are mostly useless, plus a lot of sidebar programs seem to have problems.
I use Windows Live OneCare for anti virus and spyware and its been 100% effective in keeping crap off the machine. I also do not run any sidebar programs, 3rd party toolbars, and NEVER click on any wonderful offer to fix or give me anything. Any emails that show up with links I go to the websites via Internet Explorer directly, not by the email link.
Both Linux and Mac OSs are just as inherently susceptible to viruses and malware. But the little gargoyle who gets his kicks writing that stuff can affect between 20 and 100 times more computers by writing for Microsoft operating systems than anything else. Same for alternate browsers like Firefox and Mozilla, except their tech support base is so low you'll probably have compatibility problems on many websites even without the malware.
As for Internet Explorer it has extensive built in security options under "Tools". Learn what they are and how to set it up so it works OK but keeps out the malware. Mine regularly refuses to open websites that have lost their security certificate and issues warning prompts for lots of stuff, originally too much, so I adjusted it to be a little more lenient.
And remember the ONE Commandment of Computing: BACK UP!