Sometimes if the problems you are having are to complex, it can be easier and quicker (thereby cheaper) to reinstall your operating system. Some people would just reinstall and that's that, but I will do the following:
Format the disk, install the operating system, download and install all the critical Microsoft updates (including SP2 for XP) reinstall your modem and set your system up.

I can also reinstall your printer, mp3 player, scanner etc.