I have a older system, Asus P5N (MB) Intel core 2 6600, 32 bit (OS) 4.00 GB RAM. The problem is ever since I changed from windows XP to 7. at times it will not start up at all, the fans all start but the screen is black. If I push the reset button I'll hear the little beep sound my system make and then it will start, other times it starts real slow and I get the BSOD. (blue screen of death)that say windows failed to start. After using the computer it freezes on shutdown, it gets to the screen that says windows is shutting down with the little circle that is rotating and it just stops, i have to shut it down by killing the power.
If your system doesn't even get to the BIOS screen when you power it on sometimes, that's not caused by the installation of Windows 7. Typically the cause of that is an overheating CPU, bad RAM, a bad power supply, a bad expansion card, or possibly but less likely a bad peripheral such as a USB device or hard disk.
Based on the rest of what you say, I would try to eliminate hardware issues as the culprit first:
- Download/burn a Memtest 86 iso and run it for 24 hours, see if it reports memory errors. If it does, replace your RAM. You may also try installing 1 stick of RAM if you have multiple sticks of RAM.
- Reset BIOS to defaults. Find and apply any BIOS updates if possible.
- Try booting system with no expansion/graphics cards and no other devices except main hard drive and onboard graphics only (if the motherboard has onboard graphics).
- Dust out CPU and make sure the system is well ventilated.
If you have a spare copy around, run SpinRite (www.grc.com/spinrite.htm), which may easily discard any Hard Drive issues and serve for maintenance purposes as well if nothing critical is detected.