OK. You will have to replace the power supply.
BEFORE you do this:
Review this troubleshooting guide. See if it helps diagnose the issues:
http://hardforum.com/showthread.php?t=1349426See if you can figure out the problem.
Then:
Please give the specifications of the system:
Processor, Memory (Amount and speed) Motherboard name and brand (it should be on the face of the motherboard, and Dell typically uses brand name boards.)
Why am I asking? You are about to drop 2-300 on the system. If it is more than 2-3 years old, it may be wiser to buy new than update.
Now, on to power supplies:
Simple power supply calculator. You will need to know the cpu, ram and speed of it, hard drive size and type, etc.
http://www.extreme.outervision.com/psucalculatorlite.jspTell us what it says for you to get.
Then I would look at these brands of power supplies: Antec, XFX, Corsair (typically good bang for the buck) Seasonic (expensive, high end - think server and hard core business workstations) Enermax, Thermaltake (careful with this brand, they outsource to others, and sometimes get bad models, research before buying) Super Flower (new brand, great reviews) OCZ
Careful of the cheaper brands. If it is cheap, there is a reason.
My recommendation would be to look for Corsair power supplies, as they are relatively inexpensive, and good performers. Upside: Good ratings, price. Downside: Many do not have Modular (removable) cables, and can have an issue with cable management.
Inexpensive:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139028In case you want to have multiple video cards:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139048