Major Network Issue

I have a Network of 25 Workstations, Windows 98SE, Windows XP Pro, windows XP Home using NT4 Server and NT4 Mail Server, This has worked fine for 5 years at 10Mb with Static C Class TCPIP and NetBEUI Protocols with a ISDN Gateway and Pop3 Proxy Mail. We use a Access Database Accounts Package which has worked fine for a number of years. 2 Week ago I upgraded the Network fron 10Mb to 100Mb using a DHCP Router, I removed the ISDN and upgraded to Broadband with a DLink VPN and DHCP, tunnel is working, all computer are login on to the domain and have email / internet, Excel and Word and all other applications. However the Access related database work fine on Windows 98SE but on XP Pro and XP Home it crashes with a Network Drive Error yet the Map resorses are still accessable outside of the Database, I have 2 x XP Pro 1 work but slow on crashes, 6 XP Home 1 x Works fine never crashes and open data in 12 second, 1 x open data in 12 second but crashes, 4 x take 1 minute to open data but never crash. I have applied all service patches to both NT4 Server and XP Workstations I have added fix from Microsoft Q-177266 which say to add Reg_DWORD SizReqBuf=45129 this improved 1 xp home workstation but no effect on the rest. Common between all the system failing is that they have one data point to a 4 port 10/100 hub to two computers. Follow this Server to Switch, Switch to Port Port 10/100 Hub Hub to two Computers on each table is a Windows 98SE "Work Fine" Windows XP Home "Crashes"

1. Is it XP Home / XP Pro
2. Is it Store & Forward Data Package Loss
3. Is it 4 Port Hubs

2 Week trying to sort and I have run out of Ideas.

Any pointers...

Thanks