I want to echo Bob Warren's comments. Many power supplies have multiple ratings. You need to read the ratings very carefully before you try to adapt the unit to european power. Many switching power supplies are usable on 120 or 240 volts without any modification. Some have switches to select 120 or 240 volts. Most "wall warts", little black plug in transformers, are one voltage only. Radio Shack had some universal ones that could be used with 240 or 120 and had interchangeable tips. When using a transformer, you need to be certain that it is big enough to handle the load that you are going to connect to it. Most adapters give current ratings for input and output.
Another approach is to try to get 12 volt adapters for everything that you use. I have a 12 volt adapter for my notebook, a Canon 12 volt charger for my Canon NiMH batteries and a Powerex 12 volt charger for AA NiMH batteries. The Canon and Powerex both have a 120 volt wall wart and a 12 volt cigarette lighter cord.
You can find Powerex at www.zbattery.com
If you aren't certain of something, ask specifically about that unit. It is impossible to give specific advice without knowing the details. Beware of store sales people. They are often technically innocent.