Hi,
start the PC in SafeMode (F8-Boot) and then do a normal or better a thorough scan with avast.
Note the findings exactly (name of virus/trojn, and where it was found), and best inform yourself about the found malware (and how to remove them properly ) via google or VGREP (see below)
or
move found trojans to the chest
Download Winzip, Spybot, ad-aware and cwshredder (if necessary on a different PC) and install them on your PC;
update spybot, ad-aware and cwshreedder (if possible) via their internal Updater
reboot in safeMode again, scan and fix with the above progs
if the problem is still not gone: open winzip, and open the hijackthis.zipfile with it;
extract it to a new, empty folder
run hijackthis, press "scan", then "Save Logfile"
-> copy and paste the contents of the upcoming editor here