Hi
Your PSU is not a powerful enough unit at all and that is if it is really 450W. Many no name PSUs say thast they are 450W or whatever but are actually nowhere near that in reality. You need to purchase a decent make/model that can be relied upon and a minimum of 550W if you are running a very basic machine. If you want to use a half decent graphics card or several drives etc probably 650W upwards would be appropriate.
Any of these are good quality and can be relied upon: Enermax, Seasonic, CWT, Corsair, Thermaltake, Enermax, Gigabyte, Coolermaster, Antec, and many others so you shouldn't have problems locating one. Try and find one with a single 12v rail if possible.