Is it safe to use Extension Cord to Power PC/Routers on already used circuit?
Without knowing the specifics of your wiring, you should be just fine running all of that and more from a single outlet.  On one 15amp outlet I have running all at once a PC, PS3, 120watt AV receiver, and an LCD TV -- you'll be fine!  If you do overload that circuit the breaker will trip or the fuse will blow, and you'll lose power to that outlet.

I would get a more competent electrician out to figure out what is going on with that outlet though.