Hi,
Congratulations on buying a new Windows 7 computer.
Yes, you can lock your computer while your operating system downloads updates. Locking your computer will not interrupt the installation of updates.
Good thing about Windows updates is, it uses a service called Background Intelligent Transfer Service (BITS). Which uses idle bandwidth to transfer data. Normally, BITS transfers data in the background, i.e., BITS will only transfer data whenever there is bandwidth which is not being used by other applications, for example, when applications use 80% of the available bandwidth, BITS will use only the remaining 20%. BITS constantly monitors network traffic for any increase or decrease in network traffic and throttles its own transfers to ensure that other foreground applications (such as a web browser) get the bandwidth they need. BITS also supports resuming transfers in case of disruptions.
Check the following links, you may find some more useful information.
Understanding Windows automatic updating
http://windows.microsoft.com/en-in/windows7/understanding-windows-automatic-updating
How can I tell if my computer is up to date?
http://windows.microsoft.com/en-in/windows7/how-can-i-tell-if-my-computer-is-up-to-date
Hope the above information clarifies your doubts, if you still have any questions, please post here and we are happy to clarify them for you.