My psu fan stopped working and it caused my pc to reboot every 45secs when its really hot.
On one occasion, it caused windows XP to crash (blue screen) and I couldn't do anything but format and re-install windows XP.
I've had to use my room fan to keep the temp of the psu down while I ordered a new psu.
sometimes you have to cut the 3pin plug in half to fit onto the PSU 2 pin, or get the old soldering iron out in some cases. you do need some form of electrical skills to do this. and remember some capacitors remain charged after you pull the plug. so be carefull out there.
without ventilation, the power supply will get dangerously hot! very possibly a fire hazard. i agree with leejend in leaving your system off. or tape a fan to the back (exhausting) if you need a quick temporary fix.
i'd try to replace the fan before throwing it out and buying a whole new psu. you can put the fan of your choice in (led, variable speed, etc). if it ends up being too difficult, then throw it out and get a new one.