Hi,
I've not done this before, but at a very high level, I'd do the following:
1) Build a new windows server and get it up to date.
2) Install SQL Express (same version).
3) Backup your SQL ExpresS DB on your old server. Restore it into the new server.
4) Backup your SSO and Inventory Service databases on your existing vCenter server
http://blog.mwpreston.net/2013/04/22/backing-up-your-vcenter-db-all-three-ofem/
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2057353
4) Stop the vCenter Services and Database on the old server.
5) Install vCenter software (Same version) onto your new server.
6) Restore the SSO and Inventory Service database back to your new server.
7) Check DNS and ensure records are correct, if you have any CNAMES or A records for a service name such as "vCenter.my.domain" make sure they are correct.
8) Check and test vCenter services on new server.
Happy to go in depth further or add to it based on any other opinions.