How should I travel in Europe train or by car?

I'm planning to travel in italy(venice, rome), london, switzerland, brussels, etc.

I have travelled both ways, with my own car, with rail passes, with busses, with point-to-point rail tickets, via ferries, etc. To give you a good answer--and not just guess--I'd need to know the following: Where do you need to go? What's your budget? Driver license and insurance? I'll check back to see if you added details, and then finish my answer for you.

===== Okay, thanks for updating your info. That makes it much easier to advise you.

Since you listed cities as your main destinations, I would NOT rent a car--except maybe in Switzerland. I would take train between Venice and Rome; fly Rome Switzerland; fly to Brussels; then take Eurostar between Brussels and London.

Having a car would mean not only petrol at $8 USD per gallon, but extra insurance, high tolls, hassles with maps and directions, finding parking, high parking fees, etc. Getting into the countryside in Switzerland with a car would be nice, however, especially since the largest cities are nowhere on the scale of Rome or London.

You can purchase point-to-point train tickets when you arrive to each city (Venice to Rome, for example). This site will check prices and rail passes to see if a pass or point-to-point, or a combination, would be cheapest for your trip: http://www.railsaver.com

For flights, http://www.whichbudget.com
For Eurostar, http://www.eurostar.com

I've been living in Europe for the past 9 years and travel by train or fly whenever it's possible. The train systems are relatively inexpensive and easy to use. I don't have to deal with finding a place to park and it's better for the environment. There are some places that aren't convenient to get to by train, but not a lot of them. The national train systems all have online schedule and ticket information. The German site is useful for checking schedules throughout Europe: http://reiseauskunft.bahn.de/bin/query.exe/en

Italy: http://www.trenitalia.com/en/index.html
Netherlands: http://www.ns.nl/servlet/Satellite?cid=1075985690180&pagename=www.ns.nl%2FP age%2FSuperHomepageEnglish&lang=en&c=Page
France:http://www.sncf.com/indexe.htm
Denmark: http://www.dsb.dk/cs/Satellite?pagename=DSB/Forside
Belgium: http://www.b-rail.be/main/E/
Austria: http://www.oebb.at/en/index.jsp
Czech Republic: http://jizdnirady.idnes.cz/ConnForm.asp?tt=c&cl=E5

There are a lot of budget airlines in Europe; they often have very good deals. I found a 39 euro flight from Pisa to Eindhoven for someone several weeks ago. There are a limited number of seats at the really low fares, but it's not really hard to find bargains. This is a good resource for finding budget airlines flying between a given pair of cities: http://www.whichbudget.com/

Get an Eurailpass and take the train.

Train.