Ramnit.b virus???

Figment

If a Windows disc was supplied with it, you could do as others have suggested and reinstall windows. Personally, and as they've already conceded the problems with the previous machine were their fault, I'd be taking it back to the shop for a refund and purchasing elsewhere.

WhiteChristmas

I'm betting that the shop is using a cracked installation disc to put XP onto the machines it's selling; that's probably the source.

Im_just_careful

Various people have kindly posted links to install images from the Microsoft website for their various operating systems.

Assuming the laptop has a valid licence (product key) and the hardware seems OK then this would be my approach.

fiddiwebb

If you want to try and remove Ramnit although most think it is not possible nothing to lose trying this first, obviously it depends how far gone your system has gone..........

Start your laptop in safe mode with networking.

Download hitman pro online scanner (free trial), malwarebytes, ccleaner

Turn off system restore, restart your computer/laptop, uninstall Java, run CCleaner, run Hitman Pro online scanner and remove anything it finds, run Eset online scanner and remove anything it finds, run Malwarebytes making sure you have updated it first and remove anything it finds.
Run your own antivirus (not sure what you are using?) and remove anything it finds, all this is in safe mode with networking.

Restart in normal mode and re-run the above applications and see if things are getting better or not.