I don't know if it's a bug on the upgrade procedure or what, but on two different systems I've experienced a problem on decimal and money fields after an upgrade from Microsoft CRM 3.0 to Microsoft CRM 4.0 (Italian language).
The problem appears curious: when you enter a value on a money or decimal field (for example 7) and you try to save the record, the number changes to 700.00 and it's multiply by 100 on every record save.
My first idea was to check the Organization Setting for the number format (and money format) but here the decimal separator was OK.
After different checks, I've discovered that the problem lies on the user settings table (UserSettingBase), where the decimal separator and thousand separators was wrong (both set to dot).
In order to correct the bug, you've to launch via SQL Server Management Studio this query:
UPDATE UserSettingsBase SET DecimalSymbol = ','
So strange...