Avast is a pretty decent free software.
http://www.avast.com/index
You still never said whether or not your OS is on a partition or how long you have had it installed.
You could also try a system restore if your problem has only started recently. If you know the date, choose a restore point as near as possible and click to see what software will be affected. Make a note and then do the restore. Once it has finished, start installing the software/programs that were removed. It is a slow process but may help pin point the culprit.
http://www.avast.com/index
You still never said whether or not your OS is on a partition or how long you have had it installed.
You could also try a system restore if your problem has only started recently. If you know the date, choose a restore point as near as possible and click to see what software will be affected. Make a note and then do the restore. Once it has finished, start installing the software/programs that were removed. It is a slow process but may help pin point the culprit.