Open files extremely slow

Hi everyone, since a couple of weeks, my PC (running windows XP Prof.) is extremely slow opening files independent of the format (txt, office files, etc.). To give you an idea, it takes around 45 secs. to open a 10Kb txt file. It happens only when I open the file using the Windows Explorer, and only when I am connected to the network.

It is the same in different networks. It looks like it is looking for something before opening the file.

Any ideas will be more than welcome
Cheers
Oscar