[Solved] Running 450W system on 500VA UPS for 2 minutes - wise?

I doubt that your cousin's PC is drawing more than 250 Watts.

Although Watts = Volts x Amps, "500VA" does not equal 500 Watts:
check the manufacturer's specifications for the official wattage rating.

For example, the APC 500VA UPS has a rated output of 300 Watts:

http://www.newegg.com/Product/Prod [...] 842101122R
( thus 300/500 = VA x 0.60 )

Also, if that UPS has a USB feedback cable and software to report
the instantaneous wattage output of that UPS, you should run that
program to measure that wattage.

A good rule of thumb for powers supplies and UPS battery backups
is to target 50% of their rated capacities.

This 50% rule of thumb provides a good margin for error,
and it also increases the chances of reaching the maximum
efficiency of your PSU, which tends to peak at or near 50%
utilization.

To illustrate, if your PC is drawing 250 Watts, that demand
would be 50% of a 500 Watt rating. This APC 750VA UPS
can deliver 500 Watts:

http://www.newegg.com/Product/Prod [...] 842101137R

MRFS

It will work OK, but your runtime won't be very good. It will also put a larger load on the battery and likely shorten it's useful lifespan compared to a higher amp hour battery. IMO, a 500VA is the bare minimum for just about any system because the output wattage of the UPS is roughly 60% of the VA rating. Unfortunately the APC utility forces a shut down with 5mins of battery time remaining and also doesn't support hibernating, so you might want to use the Windows power management instead.