access to network shares denied

I'm running a SBS2003 and Vista Ultimate 32 Bit SP1 on my desktop. I work a
lot with PPTP connections to other sites and remote desktop connections.
Normally I'm logged on to my desktop pc permanently. My problem:

When I log on to my desktop, accessing network shares works fine. After some
time (anything from just a half hour or so to almost a day) accessing
network shares produces error messages, eventlog shows event ID 14 Kerberos
and states that there was a password error using "logon manager" (not sure
whether this is the correct term in English, since my computer is running a
German Windows...). I'm told to open control panel, user management and to
provide a password for a certain account (one of one of the remote networks
I connect to through VPN a lot). Problem: I erased all saved credentials
already and I'm still getting the error. Also, the shares I would like to
access would require different credentials than the ones the logon manager
apparently tries to use.

I found one reference that suggested, removing the desktop computer from the
domain, deleting the user/computer account from AD and profile information
on the desktop, then joining the domain again - which is a pain due to a lot
of configurations in my user profile I would loose and it is not even
certain it would really change anything for the better. I would rather like
to a) understand, what is going on and, of course b) get rid of the error.

Best regards,
Andreas Münster