more than likely it will b low ram
Nope. Remember this PC was running windows XP to start with, the RAM didn't suddenly decrease in size over time. I've run XP on 512MB no problems at all with a few lightweight apps.
Might seem simplisted, but my advice would be to back up, reformat and then reinstall windows. If it's still bad after that then a) you've narrowed it down to a hardware problem and b) you've got all your data backed up anyway. My bet however is some form of software issue that's gone wrong, these things happen from time to time with Windows especially.
If we are looking at hardware then the hard drive failing would be the most likely situation here. It won't be the CPU.