Wont start up

my wife has a presario that had a similar problem - it would boot up, then immediately shut down. I took off the side plate and blew off some of the dust inside. that worked for a couple of days then the symptoms returned. I took it apart again but this time I removed the cpu fan and the cpu heat sink. I cleaned all the **** off the fan and the heat sink and replaced it. this time it stayed on. I checked out the power supply. compaq installed a very cheap power supply - only 250 watts. a replacement power supply for the same machine but with an output of 300 watts is available. as soon as I can scrape up the change, I'm gonna change the PS