look at this site
http://www.virusbtn.com/news/2008/03_13a.xml
it gives information about the antivirus ,if u look at it,i will suggest avg8 or avira antivir
there is something else,there is a programme called tuneup utilies,it is not an antivirus but it has something like the task manager but tells u more info about processes as location and publisher
i had a virus or a spyware like urs,open the process manager ,if u found any strange processes or by unknown bublishers or u don't know them,go to their locations,and know the name well,end the process and try to terminate the process from the process manager and then terminate its file manually with shift+delete
the virus is probably one of the running process,after dysteroying all processes manually after process manager,and deleting them manually,try changing the desktop image normally and then install an antivirus(i suggest avg8 with a rootkit scan or avira antivir)
delete all viruses and then try to work and see if it works,
i hope that this is helpful but this way sometimes work if the virus isn't too bad
and that message u recive,is it on the desktop?anyway,that is a virus,i had one like it before,it forces me to download,install buy an antivirus then this message may be removed but that won't happen
well.if that didn't work,give ur hard disk to someone who have a good antivirus on his computer and make him do a scan and try to clean ur hard disk.
i hope i was helpful
good luck man