New 2003 Server and pst's

I replaced an old DL Server running 2000 Server with a new DL385 with Windows  Server 2003 - since then, I have multiple users who cannot open their .pst's on the network share.  It's not consistent, sometimes they can open their .pst's and other times they can't. They receive messages anywhere from 'not enough server storage is available to process this request' (standard irpstack size error) to access is denied. File is in use and cannot be opened.  We run all XP desktops and the mail server is housed at an offsite 3rd party company, so there isn't an Exchange server in the mix.  The most common denominator is that everyone that is having the problem have large .pst files (anywhere from 1-7 gig).  It only seems to be with the .pst's but they receive the 'not enough server storage is available to process this request ' error even if they are trying to just move the .pst from the network to their local drive to eliminate the network.  I am waiting to hear back from one person to see if it is the 'over 2gig' size problem, to see if converting them to unicode text would help (some of these .pst's have been around for a long time and may have been created in previous versions of Outlook), but since it just started happening once 2003 server was put in, I'm thinking not?   This is a file server only, and have alot of people going to it.  It's starting to impact heavily and I'm looking for assistance.  I noted that the IRPStack Size isn't even in the registry, so I was going to try that, but trying to get a reboot on this particular server is not easy - so I'm trying to limit the changes I make and reboot as few times as necessary to fix it.  Also, what does the IRPStack actually do? Thoughts?