Uninstall a program doesn't show "installed on" date

I was given an HP laptop with vista and it is very slow to start up. I was uninstalling some programs and I sort by date so I can see the crap that was recently installed... the "installed on" date was there and now its not. It shows name and size but date is gone now. Anyone know why it went away and how to get it back? It may seem stupid but I like to know the date so I don't accidently uninstall anything that was installed by HP and may be essential.

Also any advice or programs on how to start up faster... I have run virus scan with avast, ran spybot, deleted startup entries, cleanup, defrag. Anything else I can do?

Thanks!