Computer wont start

Hello,
My brother is in the marines, so he left me his computer for a few months, it runs vista and was custom built, the grand total coming out to around $2k. The other day I got back from school, and it had been turned off due to a power outtage. I started it up just fine and used it all day, then it started to get sluggish so I restarted it, only to find that once it gets to the point where the loading bar moves across the screen, it starts all over again, completely preventing me from even getting to the login page. I have no idea what to do, I talked to a friend of mine who works with computers and he recommended a program called doctor cureit or something. So I recorded it onto a CD using a different computer, then booted it up to scan for viruses, it came up with nothing. I currently cannot find my brothers vista CD and have tried all the other boot modes (e.g. safe/last known good configuration etc.). My brother is coming back on leave in a few days, and even though he already knows, I would really like to have his computer working before he gets back, anything anyone can do to help would be greatly appreciated, I dont know if there is any free vista repair software or anything, but I'm sure someone does. please help.