hey guys, I need help and Im not sure who else to ask so Ill ask you.

Okay so Ive got an alright computer BUT Im having problems with viruses and currupt files. I have everything saved to a 250Gig USB harddrive and I just want to wipe EVERYTHING, OS and all. The problem is I dont have my OS disk and I dont want to risk downloading a pirated OS because, well, it just plan seems like a retarted mistake to make. I would realy like to keep windows as my OS because other people use my computer and there not as good with tech as I am so any way... Does anyone know where I can get a download for XP or VISTA thats trust-worthy.

I dont have the money to buy a new OS and I need your help. any ideas?