Administrative access denied in XP

Hi,

My scenario is ..

1- Windows XP(testxp) and Windows 2008 R2(testwinr2) are in the same workgroup.

I am able to access administrative share of windows 2008R2 from XP machine by using\\testwinr2\c$..But the problem is I am unable to access Administrative Share of windows XP from Windows 2008R2 by using \\testxp\c$  it is asking for user name password of Xp machine but inspite of giving the username and password of XP machine i am unable to access the administrative share of windows XP machine from R2...

Plz reply asap...

Thanks in advance...