I have a Compaq Presario SR1901 with an AMD 1.8 GHZ processor. Can it be upgraded to a 2.8 Ghz? I believe it is a Socket 939..
I need 2.8 for a work from home program and I do not want to buy another computer I have 5 already.

yes you can upgrade, check these sites out: your motherboard specs: http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00647121&lc=en&cc=us&product=184 3642&dlc
However I don't know where you can buy it from.