Spinning Circle, Explorer hangs a few minutes after bootup.

What is a "Computer parc?"

If the problem is on a network then try disconnecting from the network to test each.

If not then it most likely will be in a program you've installed or a setting. Establish a Clean Boot, run all infection checks, SFC, and then the other steps in Troubleshooting Steps for Windows 7 tutorial.

It's a set of computers (around 20 pc's).

On the network, we have a simple setup with a switch.
No crc error and each port has a normal statut (no flap).
RTT is good and constant.

I will keep you inform about further actions.