Multiuser access to XML from web service

[Ketchup Kerry]

I wrote a multithreaded windows service that has read/write access to an XML
file using a mutex.

I started wondering -- a multiuser XML file could be really useful in cases
where a small, one-table, database is needed.

Then, I am wondering how to implement in a web service.

For example, when a user consumes a web service, is each user on a single
thread on the server?

How would I construct a web service with a common resource such as an XML
document?

If I used a mutex in the web service, would each user generate it's own
mutex?

Or does the web service treat each user as a thread and the mutex as a
single resource?

Should I use 'delegates' instead of threads?