Download the 32bit and 64 bit Live CD images of Ubuntu 10.04. I am assuming that your PC has a duo core and can run 64bit as well. Try running the live CDs by booting directly from the CD-ROM. Remove the HDD if needs be. If either works well enough, it should install without a problem. If that doesn’t work out, try live CDs from OpenSUSE 11.2, Fedora 13, PCLinux or distro of your choice. Any newish distro will be compatible with the official Blender Foundation build. Blender 2.5 IS definitely compatible with Ubuntu 8.04 as this was my OS of choice until a couple of months ago. It may be some obscure piece of hardware that is causing the problem. Try remove all peripherals and add-in cards, TV cards etc and try run the live CD as close to barebones as you can. If the live CD boots, but your subsequent install fails when on reboot, try failsafe mode and if this works, it is a driver that is causing problems. You will need to isolate the faulty driver/hardware. I suggest you remove your original hard drive and install on a clean, nuked and paved hard drive. Or nuke and pave the existing hard drive with a proper low level format. Make sure that the partitioning software is set to do a low level format when the partioning is finished. When you have a working OS, install the correct proprietary Nvidia graphics drivers properly. It is not as simple as just switching on 3D acceleration in the control panel. You need to end up with a Nvidia settings application in your Gnome main menu. Check that the 3D acceleration is working. I normally check that compiz is working. Oh and check your ram with Memtest86, you can use the live CD. That and low voltage supplied by old PSUs are the normal reasons for erratic behaviour on a PC. Once you have a clean system sans cruft running on your machine, it will be much easier to trouble shoot.and get Blender running properly.