trojan called "gaopdxserv.sys" may have killed my "Windows Installer"

While trying to upgrade AVAST Antivirus to 5.0 version I got a trojan called "gaopdxserv.sys" that seems to have corrupted my "windows installer ". I am unable to installed anything. I ran Malwarebytes and delete it from the system suppously but I still cannot install anything on the computer. OS Windows XP / SP2. Please help.

Error: Windows Installer - The windows installer service could not be accessed. This can occur if you are running windows in safe mode, or if the windows installer in not correctly installed. Contact your support personal for assistance.

 

Please read this and post requested logs in the Malware and virus removal board.
NOTE:

Be aware that only Malware analysts will advise and they are often busy. Your post will be taken on a first come first served basis but it may take a while before you receive a reply.

Include a link to this thread if you can.

EDIT:
Once we've verified your system is clean post back in this thread and we can assist with your windows installer issues.