Help

I have a dell desktop...is a bit old and used but i am having trouble when i turned it on last it said my power supply was low and i had to choose windows xp (which i have always had to do) But this time it will start to boot up and then it will hit a blue screen or .5 seconds and i cant red what it says...any advice? (I just started a business and am a 21 year old college student so im very broke n cant afford geek squad or anything so any help is highly appreciated)