Clear the cmos, check the jumper and make sure there is one etc. Get the motherboard manual and see if there is a setting that can cause the problems. Clearing the cmos might let you use the keyboard you have and could remove corrupt cmos data. Reflash the BIOS (this could be the end if there is real problems with the ROM chip, so do this last).
Note that many power supplies have a second, smaller connection. This power supply connection ends in a white plastic block of four pins and connects to another connection on the motherboard. This connection is usually located between (or at least near) the CPU and the motherboard's rear ports. Make sure this power connection is also plugged in. Then check the PSU, are the voltage levels o/k, are motherboard and CPU temperatures o/k (sometimes you can does this in BIOS look for an option like health check etc). hmonitor can be used in windows to check
System components not stable
A PC's motherboard, RAM and CPU are also likely candidates as the hardware sources for boot failures, and you should give both a careful examination if you're still having trouble pinning down your boot-up problems. Prime95 is a good program for testing the CPU to see if there is any problems and memtest86 is good for checking RAM.
Other devices causing problems
You should remove as many components as possible, such as PCI cards (network cards are sources of problems as well as sound cards), extra hard discs/CD(DVD)-ROM drives and memory etc. Leave only what is need to boot into windows and see if the porblem goes away. If it does then install the hardware one at a time untill the problem occurs again. Try the hard discs first.
Also try pressing ctrl-alt-D when you boot the system (on the dell screen before you start windows). If you get the timing right, the system will run a quick 2 minute diagnostic on your hard drive as this help.
Thats all I can think of or find online at the moment and sorry if you have try this already.