Make a backup of all your important data before you start messing with the configuration of your system.
If the situation is really bad (like your virus scanner and malware scanner report lots of problems), then perhaps a re-install is the way to go.
Things to consider doing:
1. Set up different accounts for different purposes. Doing everything as "admin" is trouble.
2. Use
DropMyRights for any internet facing application (browser, email, chat etc). Run them with as few privileges as you can get away with.
3. Make sure your firewall is enabled. If you don't have multiple computers, or network printers, then turn off file and print sharing as appropriate. Windows firewall only stops things getting in. Others (eg. zonealarm) also stop unapproved things getting out as well.
4. Turn off HTML and/or preview in outlook. Spam email can get stuff into your machine even from reading HTML email. At the very least, web crumbs in such email will simply confirm that the email address is valid, thus earning you more spam.
5. Firefox + these 3 add-ons result in a pretty peaceful time for me. You have a lot more choice over what you see, and what actions the website can perform. Most sites are usable without a lot of the crud which they'd like you to have. If something doesn't work, then you can selectively enable scripts (for example) to get the stuff you actually need to work.
6. Use different browsers for different purposes. Say firefox for general browsing, and IE only for trusted banks and online shopping sites.
7. Using
virtual machine technology allows you to create machines within machines. If something does get into the virtual machine, then it's a simple matter to delete it and re-start it (compared to however many hours it would take to reinstall your primary OS). There are many such virtual "appliances" for a variety of roles.
--
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.