Before you do anything on your PC, UN-Install all the crap software that came with it.
Defrag the hard drive if you had to uninstall a lot of stuff.
Run Microsoft Update and get all Microsoft products up-to-date. This may take several re-bootings.
Then install the following (all freebies that work GREAT if kept up-to-date):
For Anti Virus: Grisoft AVG - http://free.grisoft.com
For Spyware protection: Microsoft Defender - http://www.microsoft.com/athome/security/spyware/software/default.mspx
For occasional use: Lavasoft AdAware Anti Adware - http://lavasoft.com/products/ad_aware_free.php
Manually run all of the above programs to scan for existing problems.
Schedule all but Ad Aware (must be run manually - about every other week) to automatically update Daily.
~~~ You may leave comments or questions regarding this article here. ~~~