Hi, I am thinking of buying a peavey windsor 100w head, its quoted at 100W RMS but I can't find out a what Ohm rating its for

It says it can do 4/8/16 Ohm but it doesn't say which applys to the 100w

The ohms is for the speaker/cabinet. It can handle multiple cabinet setups. You want a cabinet that will handle 100 watts and up.

Example:

4 x 16 ohm cabinets - 4 ohm
2 x 16 ohm cabinets - 8 ohm
2 x 8 ohm cabinets = 4 ohm
1 x 4 ohm cabinet = 4 ohm

etc.............