I recently came across a very annoying bug in which Vista stops access to mapped drives.

How to do it

To do it you need to alter the value of the LM authentication level to 2 in
VISTA.
This can be done by the following steps:

1) Click Start
2) Click Control Panel (not classic view)
3) Click System & Maintenance
4) Click Administrative Tools
5) Double Click Local Security Policy
6) On the left pane, click to expand Local Policies
7) On the left pane, click on Security Options
Now, on the right pane, near the bottom, click on
Network Security LAN Manager Authentication Level
9) On the drop down, change the default setting (NTLMv2 only) to
Send LM & NTLM
10) Restart PC

This resolved the issue for me I hope it works for you!