Netburst(pentium 4 based processors) are supported by basically any OS... It even runs 6X kernel, meaning 6.1, 6.2 and even 6.3 meaning you are even able to run Windows 8.
You'll need 512mb ram for vista and 1024mb for 7 and beyond.
Also DX 9.0 graphics adapter-
meaning discrete graphics from around 2004 and beyond, and integrated graphics from 2006 and beyond.
Graphics adapter is meant to run Aero glass on Vista and 7 and Blocky design(not Aero glass anymore) in Windows 8, therefore it is not needed to run those OS's, just a recommendation.
If you don't have any money, go Ubuntu or any other UNIX-like OS.
The reason you might not be able to install Windows 7 is because Pentium4 uses small adress space(32-bit). And you can't install 64-bit OS on 32-bit processor. You can do other way around, and it's even reccommended, if you have under 2gb of ram.
For example, my fathers home PC is running Windows 7 Professional(32-bit), on a Optiplex GX280 with Intel Pentium4 520.