to begin with...
go into SAFE MODE
and then try to install ur antivirus. avast/avg r good freeware.
reboot.
let the antivirus initialize and install itself fully
again reboot into safe mode and now scan ur pc.
also scan ur pc in safe mode with ewido ant-malware.
hope it helps. if it dznt, pls post ur hijackthis logfile here or at
www.hijackthis.de for analysis.