As per my experience of troubleshooting computer issues, your problem seems very common and one of the possible reasons behind this is Windows "Can't boot up".
-> It simply shows that Windows XP progress bar is working but suddenly it restarts and takes you back to POST mode.
->This happens repeatedly even though you choose to start windows normally or start windows on safemode.
->Most of the time these problem occurs when there is a sudden loss of power (ex. brownouts, power unplug) while working on windows and sometimes improper shutdown after using.
--> To tackle the problem you may try this:
->Turn On your computer and continually press DEL button to enter BIOS Setup. In case the DEL button does not work, press one of these keys F1, F2, F10 because some computers such as DELL, HP or Compaq use these to access the BIOS.
->Within the BIOS go to Advanced Features Settings and look for First Boot and make sure to choose CD-ROM.
->if you have setup the BIOS correctly to boot on CD-ROM then save the settings and exit. Press ESC key then F10 or select "Save settings and Exit" then hit Enter.
->Next, insert windows XP installer Disk and follow the instructions on your screen.
**Well i must say that your problem will be resolved by following the above steps and unfortunately if it not that means there is a serious issue....But don't worry, if Ur problem still persist you can go for any tech support company.
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FixUrSystem.com that provides instant and reliable tech services and that too at an affordable & reasonable prices.