Why does my computer lockup playing games?

Any number of reasons could cause this.

Suggest before playing game, to have task manager running with the resource tab clicked.. When the lockup occurs, you may be able to get into task mgr to see what is hogging CPU cycles.

Or, you can Restore Vista Accessories: http://www.raymond.cc/blog/archives/2008/05/11/restore-or-fix-missing-accessories-shortcuts-in-start-menu/