I presume you have windows vista as your original OS? If you have a Vista DVD (probably didnt come with the PC but it might) then you can use the restore feature which will try and fix vista for you and shouldn't lose any data.
did you back up your data? If you did then personally i would get the windows 7 disk they sent you and just do a format and full install (will be cleaner than an upgrade)
If you didn't you're in a bit of a predicament which will be hard to fix by yourself
If you need the data and you have a good friend who knows their stuff, they can take the hard drive from your machine and put it in another PC as a "slave" and recover the data ready for you to wipe it and start fresh.
I'd start with the Vista disc and try and repair.