Windows xp pc stuck in reboot loop before login screen

Hi I have a problem with my windows xp pc sp3 after it finishes loading windows xp screen just as it's about to load login screen pc constantly restarts itself
I did have cpu cooling issues so bought new cooler and rear fan also put new thermal paste in 10 mins after pc went slow wouldn't let me do anything. Pc didn't like being touched kept saying ntldr is missing took ages to sort it out then memory kept beeping took ages to sort that out but now I've got the issue where screen goes black with the cursor showing pc constantly restarts itself after windows xp loading screen when it's about to load login screen. How do I resolve this issue?
Nvidia graphics card g force 6400
cpu amd athon 3200+
512mb memory
Winfast motherboard
250gb hard drive and 350gb another hard drive