Missing OS, Partition, and drivers need help!

So this issue has to be explained with a short story of what happened to fully understand the problems I'm having...

I own a Dell Inspiron 1750 laptop with Windows 7 Home premium 64bit edition. It was running smoothly and fine up till this Friday.  My friend was browsing my laptop and saw that my antivirus was out of date. So instead of saying anything to me he took it upon himself to download AVG anti virus and got it running. Well something in the program corrupted some of my drivers or system files or something like that. It is a known issue by them, and they released a patched version since the version he had saved on his jump drive.  Well he then attempted to restore the system to an earlier date to fix the problem like they recommend.  and instead followed another series of events. starting with, the attempted restore, deleting the recovery partition of my system, installing Linux, restoring and deleting that partition, and removing windows OS, drivers, partitions etc from the computer. Basically my computer doesn't know up from down now. and I discovered this about a day late what had happened and what he did. 

Long story short, How do i fix this? Can i just buy a new copy of windows 7 and attempt to install that way? When I attempted to install Windows XP last night, it kept running into an issue with my memory and the drivers assigned to it or something. So is my computer ruined or can I save it without spending more than it would cost to replace it all together?