Just use the literal path:
myXMLDoc.Load("C:\SomeDir\SomeFile.xml)
Server.MapPath
is just used when you don't know what the root folder of your web server is. Is you are reading/writing to a fixed location you can just specify the actual path.
Your biggest problem is likely to be permissions so make sure the process your asp.net application is running under has permission to read (and possible write)to the location you want to use
And don't forget that UAC will prevent read/write from certain locations (such as the root of the C: drive)