The physical size of your PSU, some are hard to replace due to being a non standard size.
The amount of power need from the PSU don't skimp.
3 The correct connections for your equipment
1. Physical Dimensions
Besides the specs and form factors, the physical dimensions are also important factors in selecting a compatible power supply. Here is an outline of the physical dimensions of most standard power supplies:
ATX: 6x3.5x5.5", HxWxD. Most common. Uses 4 mounting screws.
Mini-ATX: 5x3.5x5", HxWxD. Rare size. Uses 4 mounting screws. Can be used in a regular ATX case, but often not the other way around.
MicroATX: 5x3x4", HxWxD. Use 3 mounting screws. Not interchangeable with ATX or miniATX.
Flex ATX: Even smaller than Micro ATX. Various sizes according to case specs; often not interchangeable.
Use the data above to determine if a particular power supply would fit your case.
The quality of a power supply can be estimated by its weight. While this is not a true scientific or thorough measurement of the power supply reliability, it is nevertheless a very simple and easy way for ordinary PC users to estimate and compare the quality of a power supply. Why weight matters http://www.directron.com/psu.html
3. Correct connections
Some boards have 20 pin connectors others 24 pin
There is often a 4 pin plug required to power Intel CPUs
Molex D plugs for IDE HDD and CD/DVD drives
SATA power connections for latest HDDs and DVD drives.
Many thanks for the advice.
I swapped it over the power units and the computer works perfectly.
I realise that for those providing assistance some of the questions might seem simple but for the person asking, like me, this stuff is really important to find out.
So, a big thanks to all those giving their time and expertise. It really is appreciated.