One bad lead can cause a computer to continue on a cycle or to shutdown or fail to detect/ boot up a computer hard drive

Test all leads that attach to your hard drive including electrical extensions,IDE,SATA

the leads from your ((motherboard to your hard drive)) make sure they have a secure connection and are not faulty or just replace them there probably old and faulty
make sure all leads that are attached to your drives dvd\cd 3 1/2 inch floppy have secure connections and are not faulty even the electrical extensions or just replace them they could be faulty a computer needs its connections to continue its cycle and to have an end so any faulty leads will end up with a computer error
also
click start control panel administrive tools computer management device manager scoll to ports(com&lpt)right click to scan for hardware changes or scroll to monitors + to expand plug and play monitor right click where you have options to update driver,disable.uninstall,scan for hardware changes or select properties where in the general you have the option to troubleshoot also in the driver tab you have the similar options when you first right click on monitor
also
click start control panel,display,desktop you can select a different desktop from the drop down list also settings you can change the settings for more or less pixels or from the drop down list pick the appropriate level of color range or troubleshoot also select advanced then general dpi normal size,larger size or custom settings
always click APPLY after making any changes
also

if your computer came with a motherboard disc the drivers could be on it

cnetdownload.com on the home page you should see the download software finder under that there will be a list platform, category sub category price from the category drop down list select drivers then from the price drop down list select free or which ever applies to your need or major geeks download this graphics driver this will depend on your operating system

http://download.cnet.com/%7Bvalue=http://www.download.com/ati-catalyst-display-driver/3000-2108_4-10608736.html

This is a WHQL release from the Release 260 family of drivers. This driver package supports GeForce 6, 7, 8, 9, 100, 200, 300, and 400-series desktop GPUs as well as ION desktop GPUs. What