well,my pc is old too,but iv recently (like 2 years ago) installed ubuntu (linux)
and its like a fresh air in a hot summer day,it needs some low spects to run ,it boot fast like 10 sek or a little more (depends on the pc but usually faster than any windows) also it offers a "app store" with 3000 + free apps and games + some commercial one ,for example "the world of goo" also in some cases you must compile the prog to make it run it and in this process you learn something new
so why not give it a try ,you have nothing to lose
p.s i must warn you ,if you try ubuntu out of a usb its damn slow ,also the flash its not good like windows