try to delete the Lightroom agprefs file. See here for information on the file location:
http://kb2.adobe.com/cps/843/cpsid_84313.htmlLightroom will rebuild the AGPREFS file when it restarts.
kung ang rereinstall ka, make sure din na pati registry is clean from LR3 software...
kung meron ka rin system wide restore, you can try that too...
isa pa question...is your copy legal?
about differences ng LR2 as LR3...medyo malaki..im sure meron readme files ang adobe site...
but the major differences are: LR3 has lens profiles and great noise reduction.
good luck.