On one of my PCs whenever I plug the power cable in or when ever I push the power switch all I get is the cpu fan starting up at unually high speeds. The speeds is so fast that its more like a vaccume cleaner!

Nothing boots up or anything, all I get is the fan.

can anyone shed some light on this?

"IF" you didn't make ANY type of change prior to this problem starting, I'd guess the power supply is in the process of dying.

The weird thing is it starts up without problems occationaly, i suppose the parts are old, it being a restoration project.

If the power supply is going south then change it as soon as you can; I’m talking from experience here (a very painful experience that is).

I checked with my newer powersupply(connected a new one and booted it up) and checked if the problem persists or not... there was no change to the booting still the crazy sequence with the fast fan.

So its not the Powersupply i can confirm.....

What is the Histroy behind the problem?
Did it just start one day out of the clear blue?
Or did you make ANY type of change to hardware?

How old is the mobo?
Might look to see if you can spot buldging or blown caps on the mobo.

A very fast spinning fan could be an indicator of a malfunctioning CPU as well so keep that in mind, but before you get another CPU try another fan first since it’s much cheaper than replacing a CPU.