Program Files Deleting Themselves

Spontaneously while using Vista, all of my programs stored in E:/Program Files (x86)/ have seemed to mysteriously uninstall/delete themselves. (Ironically, I kept most of my programs on a separate partition to avoid such problems.

This is very bizarre; Programs stored on Vista's C:/ Drive seem to remain untouched, as do other files on E:/. Although I would consider myself a fairly experienced user, I am completely lost and have no idea what's doing this.

I've stayed on XP since, worried a similar incident may affect other folders on E:/. Help!

UPDATE: I noticed that, apparently due to me not checking the path while installing the program, that Steam was installed in E:/Program Files (x86), and not in a sub folder like it should have been. Steam recently stopped working-- Perhaps a botched update is responsible for this problem?