power supply trouble...i think!

i've loaded a cd that came with my computer ( bought from a friend of a friend) who, by the looks of it has upgraded either the motherboard, proccessor or both.
anyway, the disk has (fc cd pro, 1st mainboard) writton onit and amongst via patches (??) and other utilities is a program called hardware doctor which simply monitors the hardware information - it tells you the temperature of the proccessor etc and also has  +5v, +12v -12v and -5v with a guidance of what tolerance they should be. at all seems ok except the -5v is actually at 3v which isn't right but there's no sign of anything wrong - windows 98 loads ok, no funny smells etc. if it wasn't for that little program telling me somethings wrong, i wouldn't have known.
question is, how can i check what's wrong as it doesn't say if it's the power supply end or something not connected correctly, in which case something wouldn't be working correctly??
i know a bit about electronics and could have a closer look if someone could point me in the right direction as to what could be at fault i don't have a voltmeter to hand at the moment but can get my hands on one.
any light you could shed would be appreciated.
cheers