Very much depends upon what you want to do with your PC - web browsing and word processing, programming, web development, web server, games? I'd give you different answers depending upon what your main use is.
Gaming -> a flavor of Windows (XP if low on RAM, Win 7 if more than 2G of RAM)
Anything else -> Ubuntu or another flavor of Linux, Ubuntu is very user friendly though
Be warned though that Ubuntu might require some tinkering to get working and from your question it seems that you're not really computer-savvy.
Hence my overall recommendation is the first line =)