i have a old piii no idea what make, except it has a abit mainboard.
on start up it boots then when windows is loaded it freezes for five mins,after that runs fine.
running xp pro 512 ram,tried changing ram but no difference goggling the problem suggested failing hdd.

Could be the processor is overheating or something. I recommend cleaning all the dust out of it with a vacuum (be careful not to damage it, if you have a vacuum that can blow in reverse, do that instead) ,

the pc is as clean as a new pin,having thought about it surely it wouldnt load properly or for that matter run after five mins ,or what,although googling this problem suggests hdd pages and pages of it.

You probably have a lot of programs starting up when you start windows that you don't really need. Try downloading and using this tool to disable some of the unnecessary items. It's really good if you don't really know whats required and whats not as it gives an explanation of what the items listed do and whether they are needed for system startup. Otherwise I would say you may have spyware or a virus on your pc.

Startup Inspector

There is also the possibility that PIII is a bit overtaxed... running XP pro + ?
Here's a services guide to help with reducing "overhead"

Windows XP Home and Professional Service Pack 2 Service Configurations by Black Viper

Do you get any odd errors/blue screens when running programs etc?

What processor speed do you have? If you dont know, download Everest from my signature, and run it, and it will tell you what speed it is. Also, run Task manager to see how maxxed out teh CPU is when you are running it. I had an Intel Pentium III 866mhz not too long ago, and i frequently had problems with overloading the CPU.