There are a few things I would try (see links below for additional info) Open the PC case and reaffirm the jumpers are reset to Master (some drives require removal of pins). Do not use Cable-Select.
Physically remove and replace ALL cards, unplug and reconnect all power and data cables (I know this sounds silly but it had done the trick before more than once)
Do NOT hook up printer/scanner/etc cables. You can worry about them later
Make sure your modem and/or Ethernet card are properly installed.
Plug in and power up your PC. Access your BIOS. Reset your BIOS to the default settings (this is usually a choice) and set your hard and floppy drives to AUTO Select. Reset your Boot Sequence to CD first. Shut off the machine (rebooting is usually sufficient but sometimes windows does not clear itself)
Start the PC and Boot to your Win XP Pro Installation Disk. Do NOT use a boot disk you made some time in the past or that someone gave you.
If your computer will not boot to the XP Pro Disk be sure to let us know and we can explore other options. We are assuming at this point that all components are in working order and there is a 'software" problem
During the installation I generally remove all existing partitions You may create just the primary partition and load all you programs and data there.
What I prefer is to create the boot hard drive as 2 partitions. The boot partition is relatively small and ONLY contains the computer programs and the large partition contains all the DATA files. In event of a virus/worm/etc. your DATA will be protected. For example on my 80GB drive my C: partition is 15GB and the D: partition is the rest. This is more complex and is NOT required. It can always be done later, I use "Partition Magic" in that case
Reformat your drive in NTFS format (there may well be a debate on the benefits/drawbacks on this newsgroup but we will ignore that for now)
Install XP Pro according to the program directions. On your computer it may well take a couple hours. I then do the Microsoft upgrades. This should be no problem if you own your copy of XP Pro. Service Pack 2 is a large upgrade and may take several hours on your PC via broadband.