Top reasons for random, fatal crashes in Windows XP and Windows 2000
Windows XP and Windows 2000 are both supposed to be (and typically are) much more stable than Windows 9x/Me, but there are still things that can bring down the entire system in a heartbeat, displaying the BSD (Blue Screen of Death) or simply restarting. Go over this checklist and see if any of these apply to you.

• Power Supply - a bad (or insufficient) power supply is the most common cause for random crashes, especially if you have a lot of cards, drives, or fans, or have a dual-processor motherboard. A 350W or 400W power supply is recommended if you're experiencing this problem.
• A mix of FAT32 and NTFS drives - If you have more than one hard disk, and there are different file systems on each one, try converting them all to NTFS.
• Audio Card Drivers - try removing your sound card, or at least uninstalling and then reinstalling the drivers.
• USB Hub - if you have a USB hub, try eliminating it and see if that solves the problem (especially if you have a USB-based Palm cradle and your system crashes every time you hotsync).
• Overheating - a computer will crash if the processor overheats. Make sure the CPU fan/fans are working, and that the processor temperature (read in the BIOS screen) is within normal limits. Make sure your computer case has adequate ventilation.
• Bad memory - a bad memory module can cause this problem. Try removing one of the modules (if applicable) to see if that solves the problem; rotate through all modules until you've found the culprit. Note that some computers require memory to be installed in pairs, so, for example, if you have four modules, you'll have to remove two (no more, no fewer) for this test.

Power supply
An improperly working fan sometimes causes power supply problems.  Usually before a fan stops working, it hums or whines, especially when the PC is first turned on.  If this has just happened, replace the fan or the entire power supply.  Don't operate the PC if the fan does not work. Computers without cooling fans can quickly overheat and damage chips.  The temperature inside the case should not exceed 113 degrees (45 degrees C).  Problems with the PC's power supply, the house current, or overheating can express themselves in the following ways:

• The PC sometimes halts during booting. After seferal tries, it boots successfully.
• Error codes or beeps occur during booting, but they come and go
• The computer stops or hangs for no reason. Sometimes it might even reboot itself.
• Memory errors appear intermeittently.
• The motherboard fails or is damaged.

Warning signs of a bad hard drive
These are some of the common warning signs of impending hard drive failure:

• Frequent but irregular crashes, especially while booting up Windows.
• Frequent and cryptic error messages while performing typical activities like moving files.
• Folder and file names that have been scrambled and changed.
• Disappearing files and folders.
• Really long waits to acess files and folders.
• Hard disk is silent for a long period after you request data by opening a file or folder.
• Garbled output from open files or printing.