Belkin are definitely crap - never buy power equipment from Belkin. In fact, avoid buying any Belkin. It's just overpriced generic junk.
APC is mostly decent but what good is a 280 VA unit (that's about 190 Watts into a reactive load)? What does it keep running, your desk lamp, answering machine, cordless phone system, notebook PC - certainly not a studly desktop PC?
. If you did try to support a substantial desktop PC with that think, then it is a good possiblity that you blew it out.
Trace the voltage from the wall to where it stops with your DMM - using a lamp (non fluorescent) or some such resistive load (shouldn't operate a battery backup w/o a load). Then you'll know where the problem lies.
.bh.,