nice job troubleshooting
your orginal PSU make (apevia / aspire) is probally the worst their is fro poor quality or at the very least; tied for worst !!
the antec 300 watt doesnt have enough power to run your whole sysytem by a long shot! 300 watts PSU's were used on Intel P3's back in 1999 !
You should be looking for a 550 watt Antec
Trio >>>>>>> you skin that Griz Pilgrim & I will fetch you another !!