Portal and Portal 2 are my go to tests for if a machine qualifies as "an computer". Basically, if it can run those, it's probably good enough to be grandmas facebook machine. Might have to toss an ssd into it (exactly what I did with an old core2duo hp pavilion), but generally I find it to be a good rule of thumb.
Plus it gives me an excuse to play Portal when I test a machine :P