that the PSU is anything other than a Standard Unit.
If you have never done such a job before:
Open the case > take a few photos with your Mobile Phone (so that you will know which wires plug in where) > remove the old PSU > pop along to the nearest PC World (with it in your pocket) and select a new one > If in doubt get one of the Tech Guys to identify that the fittings are the same (They should be for a similar Power Range) > Maybe get a higher Wattage, but its not essential > return home > refresh your memory by checking the photos on your Mobile Phone > install and switch on.
I appreciate all the help guys, thing is that I am au fait as far as software/hardware goes but it's just that I have never had to change a PSU so just wondered if there were any things I had to look at to make sure compatability wasn't an issue when selecting a new one. I know that I have to check that the correct amount of berg, molex etc cables were present on the new PSU but that was about all I knew. I now know from you that I must make sure the wattage is the same or higher - anything else I need to know before moving on guys ?
check that the psu has a 20+4 motherboard power connector, some new ones have just 24 meaning buying an adapter. with a 20+4 connector you just tuck the 4 pin part out of the way if you have a 20 pin connector motherboard.