It could be the power supply or that there is a critical error in Windows, either way it's cheaper to fix both of these issues yourself rather than getting a shop to do it if you are willing to give it a go. You can order a power supply and a new copy of windows online (you may have a back-up preinstalled though if you bought the pc new). I order from play.com or ebuyer.co.uk.
First I would take the side off and have a look inside and see if the fans are chocked with dust (the ps fan is at the back of the pc at the top and is a grate with holes) Give them a hoover (carefully) if they are dusty and see if it helps, you can get cans of compressed air from computer shops to give them a really good clean too.
If still the same issue I would replace the power supply, you can see which one is in there already from a sticker which is on the back or side (if it's on the side you can take it out easily as they are just screwed onto the rail inside and slide out) The connector cables are easy, they only plug into one place on the board (and you can see where the old ones are plugged in) so you can't get it wrong. Power supplies usually cost about £20 or less (unless you are a gamer and need something with more oommf).
I got my Windows 7 from Ebuyer.co.uk for about £70 and it's super easy to install, just back up your stuff onto disk or another hard drive if you have one, put in the windows disk and follow the instructions, takes less than an hour.
My PC is left on for most of the day, even when not in use and I find that my power supplies last 2-3 years (I have a gamer grade one as I sometimes play) Even with regular cleaning etc so it's most likely this. But I don't usually get the blue screen when they are failing, I get random switch offs even when in use. It's always worth reinstalling Windows now and then though to keep it running super sweet and always worth having your own copy of windows on hand just for the repair function which can save you a reinstall sometimes.