I have an acer f672cr mobo with a e2140 intel processor dual core. Could anyone recommend a faster processor?

I want to upgrade my processor if poss without changing stuff like psu's or extra fans, is this possible? My system runs vista hp and its so slow even with 4gb of ram i have which i know only 3- 3.5 gb is used. I don't use it for gaming just office stuff. Its been slow since i bought it. Its doing my head in. Thanks in advance for any replies.