I have a Medion tower PC Pentium(R) 4 CPU 3.00GHz 1.00 GB of RAM?

I am running Windows XP on this machine, I purchased it from Toys R Us about 3 years ago. When I switch computer on, it wont boot up it either freezes showing windows logo or just a grey screen, I end up having to switch power off on back of tower. after switching it on, then going through the same process 6 to 8 times it finally boots up. How can I overcome this problem, I keep thinking it will never switch on again, so sometimes I switch the monitor off and leave it on for a week, but its performance then seems to get slower and slower. I shut it down for a day then have to go through the same process again.