For quite some time I have had an old WinXP server, with a couple of shared folders, and a Win7 client, accessing those shares.

It used to work flawlessly up until 2 weeks ago, when there was a black out and no batteries. The WinXP server came back up just fine, but for some reason all network settings on the Win7 client were wrong. I fixed them, but the Win7 client wouldn't mount the winXP shares anymore, but keep asking for some network password. Google told me it is a smb version problem. Indeed, I was able to fix the problem by creating an smb2 entry under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters and setting it to 0.

On Friday, we had to power down the systems again, gracefully this time. We started them up this morning and now the problem is back. The Win7 Client cannot access the WinXP shares. However, the configs on the Win7 client are ok, including the smb2 setting in the registry.

Now googling is very difficult, because it keeps suggesting me smb2 problem-fixes, but this issue should be solved. What else can I do to make Win7 access WinXP shares?

Assuming W7 is the client and XP is the server

When you have there an SMB2 issue, the mentioned setting won't fix it.

Enable Client-SMB1 on the W7:

sc.exe config lanmanworkstation depend= bowser/mrxsmb10/mrxsmb20/nsi 
sc.exe config mrxsmb10 start= auto
net start mrxsmb10

If doesn't help, disable Client-SMB2 on W7:

sc.exe config lanmanworkstation depend= bowser/mrxsmb10/nsi 
sc.exe config mrxsmb20 start= disabled

Quoted from Microsoft

The problem was that the clock on the XP server started to lag behind and, starting from Win7, Windows won't authenticate if the clocks differ too much.

It appears the clocks don't have to match precisely. I just set the clock on the XP system to the time my mobile phone showed and that fixed the problem.