Hi I have just fixed up an old PC by buying a Chaintec MB Via Apollo 133 Pro chip set and a PIII 600 Mz cpu for £20 off Ebay I all ready had 256 MB ram and 20 Gig HD. Put XP Home on it and it runs like a dream not as fast as my 3.2Ghz but for £20 can't grumble I am using it right now.
As Fruitbat said if everything goes alright then a couple of hours but I would set aside an afternoon.
Back everything up that you want to keep to disk or another partion or drive.
Make sure you have all your drivers to hand and I always download the latest definitions for my anitvirus and malware apps and put them on disk as well. That way once you have finished the install av back on first thing and update defs before you have to go anywhere near online.
Do a clean installation as fruitbat said.
This thread is now locked and can not be replied to.