I've been having this problem with a network I maintain where users will complain of going to open or save a document in Word, and it freezes up, and it happens to anyone trying to do those functions simultaneously. Obviously, it's not just Word--it just is the most common scenario. After a minute or two, the application becomes unlocked and they can continue working.
This only happens on their Windows XP workstations, when accessing files on the Windows 2003 Server (for Small Business).
I've verified that if you try to open a network share from the server during the period when this happens, Explorer will freeze too. I'm assuming there's sort of a response from the server, but not a complete one, since it doesn't time out or tell you that the resource isn't available.
This seemed to increase after the workstations were upgraded to Windows XP SP2.
About this setup:
The server is running Exchange, DNS, Routing and Remote Access, and sharing files and printers for this small business with ten or so computers. The server's hardware specs are well beyond the recommended specs provided by Microsoft for this, has plenty of free physical memory, and does not produce high processor usage errors (I have performance monitoring on and set to send me e-mail if anything odd happens). Also there is nothing in the event logs of a suspicious nature when this happens.
The network has one high speed Internet connection with static IPs, and is connected directly to the Windows 2003 Server, which, through Routing and Remote Access, is assigned a range of IPs and provides NAT and firewall on this connection and shares it to the entire internal network, connected to the server through a second NIC. Very little outside of standard routing is done, with exception to it statically outing RDP requests on specific IPs to specific computers so users aren't forced to always use the Remote Web Workplace.
Between the server's NIC assigned to the LAN (using 192.168.0.1, of course) and the workstations is a simple 24-port Linksys 10/100 switch.