Every time some blue screen that explains any sort of error, the computer restarts. I mean that was more than 20 or 30 times we have. What are possibilities about problems in Windows XP? Help me!

You've just described the most common problem with Windows that could have thousands of possible causes. It usually indicates a hardware failure. Maybe your memory has gone bad, or your hard drive has a few bad blocks, or your motherboard is malfunctioning, or your video card is. Could be anything. If you've had that computer for a while, try disassembling it and vacuuming it inside (be careful not to touch anything). It's not a joke; sometimes vacuuming all that dust inside can solve the weirdest problems.

If it's a recently purchased computer, then take it back to where you bought it from. Let them handle it.

We actually replaced the hard drive with a new one. But it sometimes still have problems

I say back up the data you have on it and then restore it. If that doesnt work you are screwed.