I have 3 PCs on a network (2 wire modem, router, and switch).  All were able to connect to each other and share files with no problems, and now they can connect to the internet but not to each other.  Nothing in the physical network (cabling, router settings, etc.) has changed.

PC1 is Vista Home Premium and hosts our QUICKBOOKS and Xactimate network versions and stores our shared files.
PC2 and PC3 are both Vista Business.

All computers have the same users/passwords configured on them.  All have network discovery, file sharing, public folder sharing, and printer sharing on and password protected sharing and media sharing off.  All have the same workgroup and are set to "private" network.  All can access the internet with no problem.  All have folders shared. 

PC1 can access files and folders on PC2 and PC3 without any issue.

Neither PC2 or PC3 can access PC1 unless they are in safe mode with networking support.  I have also tried a "clean boot" using msconfig and neither can connect to PC1.  When attempting to connect from the network map I get a log on box.  When I enter any correct username or password for the accounts that are the same on both machines, I get "logon unsuccessful..."  (Before it would connect without even asking for a username/password since I am logged in using an account that exists on that MACHINE).  When I attempt to connect using the previously MAPPED network drive (which is showing a red x as disconnected) I get a logon prompt as well which says "Unknown username or incorrect password."

I have tried a lot of suggestions that I have found for people having similar issues, including changing the local security settings\local policies\security options\Network Security: LAN Manager Authentication level to "send LM & NTLM - use NTLMv2 session security if negotiated" (done in the registry on the vista home machine).

All 3 PCs have McAfee Security Center installed and I have tried disabling the firewall and McAfee completely with no success.

Any help would be much appreciated.  I really need to get everything in working order again.Did you happen to disable McAffee on all 3 PC's and re-boot them ? ?
It has to be something in the new security settings since you stated it used to work...Yes.  Same thing happens.  I actually did a clean boot with nothing but microsoft services running at startup on all 3 and still can't access PC1 from PC2 or PC3.  It seems like it must be a Vista thing since it keeps saying incorrect username/password, even with a clean boot, but works in safe mode with networking and the vista business computers can't access the vista home computer.If it works OK in safemode with networking then something has changed on 1 of the 3 PC's...
You MAY want to check all 3 and see whats loading at startup...and 1 or more of the 3 may be infacted...
Thats where i'd start at least.Turns out that one of the routers or switch was causing the problem.  I cycled the power on the the 2wire and the router and the switch and now everything works!  It was probably the 2wire because i had reset the other devices previously with no result. Good news INDEED...