can't make unity session in 11.04

i was using 10.10.it was working fine.Then i download 11.04.Burned the cd
installed it on my acre 5742 it is working well unity is great addition
but
i tried on my desktop computer with out installing it, a dialog box appeared saying some thing like that my system is not compatible to start unity session
i bought nvidia quardo fx 4000 only to use unity
but now it simply starts classic session why can't i start unity session??????? i logged out from live session and selected ubuntu (not classic desktop) but there was no unity.why????????????????????? also i checked for proprietary drivers but there was no driver avaible
by default when some one tries ubuntu 11.04 without installing it , it starts unity session as i saw in my acre5742
?