.net framework keep older version?

In my add/remove programs I notice I have .net framework 2.0 sp1, 3.0 sp1, and now 3.5.

I was curious, am I supposed to keep all of them on my computer or is it possible/ helpful to remove the older versions and just keep the 3.5? If I do remove them, I assume I have to boot into safe mode before I can take out the older versions?

Yes keep them, they are all required.

If you remove them all and reinstall only 3.5 SP1, it will install 1.1 (I think), 2.0, 3.0 and 3.5 with all hotfixes/service packs.