When you're running Windows in Safe Mode with Networking, only Windows and any of the barest of necessary drivers are running. That means that the problem does NOT lie with any of those but with something else that is running when the computer is running Windows in normal mode.
The next step is to continue with the process of elimination, disabling a few startup programs in msconfig and then test.
If still slow, then something that is still running is causing the problem. So you would re-enable those that were disabled, disable a few more, reboot and test again.
If it's now fast though, then something that you just disabled was causing the problem. So, enable a couple of those that were disabled, reboot and test.
You get the idea . . . Keep on testing like this until you zero in on the program causing the problem.