yeah. that happened all the time when i had Uni and NL expansions on my old laptop. i bought a new (better) laptop (because i dont have space for a desktop) and it doesnt happen anymore, and i've installed OFB and Pets on top of it all.
The only time it has happened to me with this new laptop was when i had three other games installed at the same time so there was less free space or something. i think it just overheats or something.
And cape, apparantly they are releasing a cheat where u can make the pets selectable. Alternatively, you could type:
'boolProp testingcheatsenabled true' in the neighbourhood screen, enter the household, click on the pet and click on "make selectable"
Then type boolProp testingcheatsenabled false and voila!