antivirus

i have a PC which is virus infested so is it good to install kaspersky and then update it so that i can scan and remove all the viruses.or should i just format the whole HDD(i dont want to lose everything by format it ) and then install new win7.

reformatting is probably the better option if theres no AV already on there. Quite a lot of the bad stuff out there will try to prevent AVs being installed.

actully there is an outdated kaspersky in that system , so there wont be any problem with it ???

update it and see what it can do.

but its validity is over thats why im goin t buy a new 2013 version

I personally have NOD32 v5. What's good about NOD32 is that it doesn't have software updates every single year. And if there's one, you don't need to re-purchase a licence for that new version. It's pretty efficient too.
For your viruses, backup your files, do a full format (not a quick one) and re-install Windows using your licence. (I'm for downloading stuff but I'm against Windows downloading).

there is no need to do a full format, a quick one will do the job. And the big flaw with nod32 is it costs a lot more than kaspersky. Even though nod32 is the better AV, kaspersky is very good and perfect for people on a budget.

If your Win 7 is a legal copy, Microsoft Security Essentials does a great job.I'v been using it for 1 and half years now and not a single virus corruption as of yet.