Things for my computer?

Hi guys, im building an M-ATX PC for s part-time server and music production PC, ive already sorted out the prosessor (Sempron) and mobo (Some amd stuff) and sum ram (2GB crucial shizzle) and a random case, basically, this comes to around 100 or 90 quid ish, i just need os, hdd any suggestions for anything like, less than 30 quid