How to get rid of a computer virus ?

Comp. is only about 3 months old. Has the latest Norton Antivirus and Webroot Spysweeper and good cookie cleaner and so on. I thought it was basically virus proof. But I have been told I have a virus and it will cost $249.00 to repair. Computer is acting haywire and I don't know where to start.

I have already reset system to a previous date but didn't help.

Any advise ?

How do I find anything out without taking it in ?

Additional Details

1 minute ago

Anytime I run Nortons or the SpySweep It says no threats or viruses detected.

But the comp. is only running good for about 15min. at a time.

Until now I have never even had to re-boot before.

How to get rid of a computer virus ?internet security software

Lol you MAY NOT need to re-format your computer.

How-ever the Following programs will take hours 2 compleate

If u can get on the net I suggest u download Bullguard which give's u a 30 or 60 day trial!! I have this program meself.

Get RID of Norton as soon as u have downloaded bullguard, install and then run!! The scan will take a while 2 compleate.

It takes about 1/12h to fully scan my system!!!

Once the Viruis scan has run you will proberly find that bullguard is unable 2 remove a virus or 2. this is becasue some viruis lodge them self's in active programs. If the program is active Bullguard will not b able 2 clean it.