To start in Safe Mode, rapidly tap the F8 key after the manufacturer's splash screen goes away (and before the Windows splash screen shows up). That'll get you a menu where Safe Mode with Networking is one of the choices.
If the first entry on the list says something like "Repair your computer" - go ahead and try it (Startup Repair) and see if it helps.
I suggest using Safe Mode with Networking so that you can connect to the Internet on the problem computer. If that doesn't work, then try the plain "Safe Mode" entry.
You can also get started on diagnostics by using these free, bootable programs (they don't need Windows in order to boot up): Link Removed Please post back with the results.
Post back about your experiences...