finding a file in a class

Hi

I am an asp person so very used to using server.mappath

now i am wanting to write an xml-xsl transform that takes place in a class

can anyone tell me how i navigate to my xsl file which is a file on my harddrive at the mo

i have tried server.mappath and also the place on the hdd eg c:\e\blah.xml

nothing works?

anyone help me?

thanks

sarah