Hi, there is your CPU a 1400mhz athlon (NOT athlon xp) or lower?, it just so happens that i have the same motherboard, and i tried an AMD athlon 1200mhz on it and when you go to install windows xp, or try to boot windows up it just reboots, like it doesn't like you!, well if you are in the same boat, which i think you are then you need to immediately remove the cpu and stop playing with it, because the 1200mhz i had on it fried itself after about 20 tries!, But There is a solution, an amd athlon xp 1800+ works just peachy so anything above or maybe slightly lower will work!, currently on Trademe there is a number of amd athlon 1800+ cpus for sale, follow this link, # http://www.trademe.co.nz/Computers/Components/CPUs/AMD/auction-182065118.htm# excluding the hashes, i hope this helps.

Check all your fans and heat sinks including power supply one
Also check CPU heat sink for fluff and dirt