Delay in all activity across network

I have a strange problem in windows xp pro.
We recently deployed Windows XP pro in our organisation.
Now almost every action across the network gives a 10 to 30 second delay.
eg: Opening excel and opening an excel spreadsheet that on a network share gives that delay, while double clicking the sheet opens it directly.
These delays also occur in internet explorer (when starting it), opening my computer (with mounted network disks) and mostly all other apps/docs that use the network.

I hope there is a sollution for this.