I am going to move from Windows to Linux, i have used Linux on and off for a long time but my main issue with windows is that its so easy to bypass its security. However on creation of a ubuntu/mint user account on Linux, how secure is this compared with a account on a windows machine? Will my data really be secure in the event of a lost or stolen laptop etc?
Your data on Windows machine would be the same safe if you used full drive encryption. In Linux you can encrypt home folder during user creation, something that Windows does not have build in to OS (but there are tons of software, freeware, shareware, payware etc that can do it) It really makes no difference what OS one is using. The weakest link is ALWAYS the user.
I'd choose Linux, just keep in mind if you choose to encrypt the home folder you'll be entering two passwords every time you boot up.