Why would you need to buy a Dell to use your current monitor and mouse? Dell doesn't use proprietary connectors. My home Apple PowerMacintosh runs a Dell UltraSharp 19" monitor I have had for years. No problem.
When in doubt clean it, run antivirus software, last case reformat the hard drive. Killing it is painful but better than buying a new one. The new windows works fine if your computer can handle it, if not buying XP on ebay is not a problem.