When trying to troubleshoot a Windows XP boot problem, there are a number of different techniques you can employ before admitting defeat at your local computer repair shop. First, go for the easy ways that can potentially fix a typical Windows XP boot problem. If you can still get your system to turn on and run, albeit not quickly, try using the system restore feature to attempt to bring your system back to an earlier and less troublesome point in its history. If you still experience a Windows XP boot problem after this, try using the last known good configuration tool. Simply restart your computer, and hit the F8 key when you hear the startup beep. This should bring you to the Advanced Options menu, where you can select the last good configuration option.
If neither of those techniques fix your Windows XP boot problem, your best bet is to run Windows from either the startup disk that came with your system, or from a backup image disk that you have on hand. Slip your disk of choice into the CD or DVD drive on your computer, and reboot. You should be able to work from the disk, albeit more slowly than a typically installed operating system. This should buy you enough time to finish any important work you had before the Windows XP boot problem ravaged your system, and possibly avail you of the internet access you need to troubleshoot the problems on a more advanced level.
If your Windows xp boot problem is not alleviated after trying all of the above techniques, you might want to consider bringing your machine in for an evaluation at your local computer repair shop. There might be some deeper issues in your system that only a professional can fix correctly.