Your computer is slow because it is a slow computer. If you switch to a faster computer, then it will be faster, whether you are running Windows, Linux, or Mac OS. Here are some suggestions how you can find a faster computer for free: https://kmandla.wordpress.com/2010/1...-old-computer/
Right at this moment in history, Windows XP is about to become obsolete, so companies/universities are dumping all their old XP hardware. I bet you can find a nice dual core system with 1gb ram if you ask family/friends/coworkers.
Anyway if you really are stuck with the Pentium 4 with 256mb RAM, and there is no option or possibility for upgrade, then my most enthusiastic distro recommendation is: antix
Good luck and welcome!