The following programs can scan your PC for vulnerabilities.  All are free. Some require registration.

Antivirus

• ClamAV:  This is an open-source virus scanner
  
• Grisoft AVG Antivirus Free Edition
  

Hardware and Software

• Aida32:  Provides summary and detail information about the hardware and software installed on your PC plus performance benchmarking.
  

Spyware/Adware

• Spybot Search & Destroy
  
• HijackThis:  This is an advanced tool that can help with problems other scanners cannot.  But use it with extreme caution - it will also allow you to destroy needed parts of your system.
  

File Cleanup

• Heatsoft Clone Cleaner Lite identifies duplicate files.

TCP/IP Ports and Vulnerabilities

• Local Port Scanner works without a network connection
  
• ShieldsUP! is a simple web-based scanner
  
• Tenable Network Security's NeWT is a thorough port scanner and vulnerablity scanner.
  

Memory-Consuming Processes

• Codestuff Starter identifies the programs that run at boot time, including startup folders and registry.  It allows you to disable a program then restore it if you change your mind.