Infection Issues

Hello-
I have been plagued with erratic internet speed, browser glitches, and just strange computer performance in general. Last night, I decided to run a full scan with my avast! Internet Security 2012, and it found an infection for the second time in a week. Here is what it found:
File name: C:\...\A0046376.exe
Threat: Win32:Hupigon-ONX [Trj]
avast! successfully put it in the chest and all seems fine now. Personally, I do not use illicit sites, however, I do spend a lot of time on Facebook, StumbleUpon, and Pinterest.
Of further note, I have noticed some services in my Windows XP Professional SP3 x86 that are not in the services catalog at sites such as Elder Geek and Black Viper.
When I shut the pc down at the end of the day, I notice a thin, blue strip briefly flash at the bottom of the screen where the Task Bar would be- it has a logo that reads 'AUSmotive.com'. I have never been to that site intentionally other than to investigate what it was about. Apparently it is an automotive enthusiast review site.
Now, I am like really paranoid about all of this stuff. Is there someone here who could help me be sure my pc is free from hijackers, malware, etc.?
Thank you.