why my pc runs slowly

What factors slow a PC. Mine is a Acer 7200 Veriton, 512 RAM, 1.7 Ghz, P4, has 60GB HDD of which 1/3 is empty. It runs Windows 2000, is one year old, has been disk cleaned ans defragged regularly, temp files emptied, recycle bin emptied but STILL LOADS SLOW AND RUNS PROGRAMS SLOW. In Photoshop jams for 5-10 mins at a time. Reinstalling made no different.

Suggestions please?

Thanks
Bruce