Microsoft Fix It Solution Center - It doesn’t get much easier than this: Select a device or program, select a problem area, then browse the tips and programs that Microsoft offers as a solution.
PCWorld - Articles seem to be kind of dated but still some good information in here
Microsoft at Home - Fix common PC problems - This guide from Microsoft covers the basics for keeping a healthy PC, plus tacks on some tips for locating problems. Reading through this should let you know if you have a software, hardware or connectivity issue and give you the smarts and apps to tackle it.
ESET Online Scanner - Free online Virus Scanner from ESET
Kaspersky Online Scanner - Free online Virus Scanner from Kaspersky (has been offline recently, good option if it is working again)
TrendMicro Housecall - Free online Virus Scanner from TrendMicro
Malwarebytes Anti-Malware (Download) - Very good malware scanner and removable tool. The official website is good, however they usually forward you to CNET’s Download.com which has started to do some very spyware-like practices themselves so I recommend downloading from FileHippo instead
SIW - SIW, System Info. Shows hardware and software information, very handy