How to remove the One OS, if more OS are installed?

I had Windows XP installed on my PC but was making lot of problems like low processing and virus infections.So i decided to install the new windows. Unfortunately i selected the install fresh copy instead of formatting of previous version. During installation one error message appeared that some file are missing, I just pressed ok, then it appeared 4-5 times. I clicked ok again and again. Resultantly the installation stops. When i restarted the PC the previous installed windows was working but the newest one is neither working nor formatted. I just want to remove the newest one. Please help me out.
