It is a good idea to keep more than one copy of anything you wouldn't want to lose (especially around deadline time). Files kept on the MCS are automatically backed up by the University Information Services. If you accidently delete one tell them as soon as possible - the quicker you do so the more likely you are to get it back. You can connect to your MCS file space from your own computer and copy files over. You might also want to keep backups on CDs, DVDs or USB flash drives, all of which you can buy from the Computer Office. Make sure you take regular backups, since computers don't always give a lot of warning before they die on you, and accidents such as dropping coffee on a laptop or putting a disk through the washing machine are not unknown.
Sadly computers do get stolen in Cambridge, but there are things you can do to minimise the chances of it happening to you. Lock your door when you go out, even for a few minutes, keep your computer out of sight when you are not there and especially don't leave it near an open window (even on the first floor).
You might want to invest in a padlock and cable. You can get ones called Kensington locks designed for computers, and though they aren't cheap they are a lot less than the cost of a new laptop. There are also applications and websites that help you to track down your laptop if it is stolen. You will probably need to register your computer and install the application so it's best to look at these while you still have it in your possession!
Remember also to keep a backup of your important files somewhere other than in your laptop bag, if you have one - it has been known for a thesis and all copies of it to be carried off. Your data is worthless to a thief but it certainly isn't to you.