Cannot Execute Xp_cmdshell. Access Is Denied While Copying Data Over The Network

Hi,

I am having
some trouble copying data over my workgroup network from my Windows
2003 Server Machine (machineA with SQL SERVER 2005) to one of my
network Machine's drive(MachineB).
Here is the T-SQL code that I am trying to execute:

EXEC xp_cmdshell 'copy D:Datafile.txt  \MachineBDocuments'

Whenever
I tried to execute the above piece of code, I get the error message
"Access is denied", but if I try to copy the file from the Command
Prompt (cmd.exe) with the copy command, the file copies fine over the
network.

I have already searched over the internet and I found
out that loads of people have the same issue, and they were suggested
something like this:
"Check in Services and make sure that the MSSQLServer service is run as
a domain user and that domain user has rights to these network
resources."

Well
it sounds plausible, but I don't know what are the exact steps to do
this. How do I know which user is running the MSSQL Server service? Are
they referring to the user which I use to connect to my SQL Server
Database engine throuhg the SQL Server Management Studio?
Also they are suggesting 'domain user', and as I said before I do not have domain network just regular simple workgroup network.

Here
are some details of the user that I use to login. I generally login
into my Windows 2003 Server machine with user called 'User1' and I use
the same 'User1' to connect to SQL Server through the Management Studio
Screen.
Should I create a user called 'User1' on my MachineB(Destination Machine)?

I would really appreciate, if someone can give me detailed steps explaning how to solve this problem.

Thank you very much once again.