DELL INSPIRATION TOWER TURNS ON AND OFF - Dell Dimension 4700 PC Desktop

If the PSU power supply units fan is not working the PSU is faulty

http://pcsupport.about.com/od/toolsofthetrade/f/powersupplytest.htm

Test all power and data leads that attach to your hard drive IDE,SATA
the leads from your MOTHERBOARD TO YOUR HARD DRIVE make sure they have secure dust free connections and are not faulty or just replace them they could be faulty

motherboard and a hard drive any leads between them will fail before your motherboard or your hard drive if its a flat ribbon 40 pin type IDE replace it this will be the first to fail

make sure all leads that are attached to your drives dvd\cd floppy have secure dust free connections and are not faulty or just replace them they could be faulty a computer needs

power and data to travel through every working device and to continue its cycle and have an end so any faulty leads will end up with a computer error

even something as small as a faulty power lead or fan lead can cause you problems

computers need all of the data and electrical current to travel through every working device and to have an end to be able work properly

make sure ram modules and cmos batteries have dust free secure seatings and cmos battery has charge some motherboards cmos batteries are soldered in

you might not have enough computer ram

not having enough computer ram will run your computer over its limits putting to much strain on the CPU central processing unit and can cause memory dumps

you might have to many programs trying to or accessing the internet at the same time
scroll to bottom toolbar (bottom of screen) right click select task manager

applications you will see whats running
if you see anything you dont need running select end now and you might uninstall that specific

program if you or computer does not require it to run and
some unnecessary programs that could be jamming up your computer

also under processes see what is ticking over in the background again if you see something you
dont need to be and users you will see who is using your computer

you might need to add more computer ram
if you're not sure if a module is right for your system use the

Crucial Memory Advisor tool for a list of guaranteed compatible modules. first you will need to select the manufacturer from a drop down list then select your product line

from a drop down list then select the model then click find it will take you directly to compatible ram also you will have an option to scan your system
you should always ask for a guarantee on the compatibility of the ram before making any purchase

might be getting hot

check you CPU make sure its securely seated and has thermal grease

http://www.hardwaresecrets.com/article/Thermal-Compound-Roundup-February-2012/1490

hope this helps