Real-time protection:
You are using the free version of avast 7 right?
Didn't use PrivateFirewall for a long time, did they improve their whitelist?
Do you have Java Installed on your system?
On-demand scanners:
VTUploader (Free) -
link
To upload a file to VirusTotal, you can visit the main analysis site, click the Browse button to select a file from your hard drive, and then click the Send file button. You can make this process even easier with the free VirusTotal Uploader utility. After installing it, you can simply right-click any file under 20MB and choose "VirusTotal" from the Send To Windows menu. The scan results will display in your browser as usual.
Virtualization:
I strongly advise you to install Sandboxie and use it for when you're browsing the Internet or running shady/unknown programs(not that you should do that but...
) . Alternately you can try
BufferZone PRO (Free) , another great virtualization software.
Sandboxie (Free/Paid) -
link
Sandboxie runs your programs in an isolated space which prevents them from making permanent changes to other programs and data in your computer.
Additional browser add-ons
Ghostery (Free) -
link
Ghostery is your window into the invisible web – tags, web bugs, pixels and beacons that are included on web pages in order to get an idea of your online behavior.
Ghostery tracks the trackers and gives you a roll-call of the ad networks, behavioral data providers, web publishers, and other companies interested in your activity.
You can read
here a review on Ghostery
Extra protection:
EMET 2 (Enhanced Mitigation Experience Toolkit) (Free) - Optional (Recommended mostly because your are using Windows XP ) -
link
EMET provides users with the ability to deploy security features built into Windows to arbitrary applications. This helps prevent vulnerabilities in those applications from successfully being exploited. DEP, SEHOP and ASLR are such security features, if you want to learn more about what they do, you can watch this
video from Microsoft TechCenter. To configure EMET you should follow this
guide by rationallyPARANOID.