Srinivas,

I think there should be a good article on PC--Microcontroller serial communication on this website.

Most of such problems, as you stated, are due to lack of proper ground on PC. I have even seen the PC hanging, and sparking (in dark) when such a system is connected to an un-grounded PC.

Opto-isolation of serial conmmunication is not difficult. With a little search, you can even find some ready made and tested (as claimed so) circuits.

Try grounding your PC as a first remedy.

Good Luck

IA