Are there any free virtual browser which can bypass the office network?

Well it depends on what you want to do. If you want to look at a site that is blocked I would recommend going to http://www.btunnel.com or http://www.vtunnel.com and then just enter the URL you would like to go to. This will probably be your best option because it masks the URL to go through the proxy server/IP filter. Hope this is what you need and good luck.

As for the USB post above mine. Even though you may run the browser from a Jump Drive. The information is still being sent through the network or proxy server. The network administrator will still have a log of what sites you visited and the content or URL. The website will probably still be blocked. The only thing the jump drive does is store the temp files and history on the jump drive. All the bandwidth that you are using is still going through the network.

Here is a link for a portable version of firefox if you would like to try it though:
http://portableapps.com/apps/internet/fi...

All of them can and can't... bypassing your office network is just a matter of outsmarting your administrator, not of finding a particular browser.

you should get usb memory stick and put a browser on it and load from stick even the most basic office pc will have a usb port and this is a very cost effective and safe way of doing this because 1. 20 bucks will get you half a gig and 2 there will be no record of it since the information isnt on the computer

Tor and the FoxyProxy extension for Firefox can break through blocks. It slows the Internet down, though, so you only use it when you have too. If you have the technical knowhow to set it up, you could easily change it between ports to beat the network administrator every time he thinks he's blocked it.