Hello krAnand,
Could you please be a bit more specific about the error that you are receiving?
Would it be possible to attach a screenshot of the exact error that you get when trying to launch QTP11?
Also, as I understand from your post you have had UFT11.5 installed on the machine and then you decided to go back to QTP11. Is this correct? Have you uninstalled UFT11.5 and then tried installing QTP11 on the same machine? If this is the case please note that there is a known issue when you try to install QTP11 on a machine that had UFT11.5 installed. You can find more information and a solution for this here: http://support.openview.hp.com/selfsolve/document/KM00452328
Finally, I would like to inform you that this is QC/ALM support dedicated forum , so in future if you have any queries regarding QTP/UFT, please post your messages here: http://h30499.www3.hp.com/t5/Unified-Functional-Testing/bd-p/sws-Fun_TEST_SF#.Uo9DuMTdewk
Thanks,
Hristo