How to join the updated result set of multiple servers
Something like this should do it:

create table #Tmp
(
  Drive char(1),
  MBFree int,
  ServerName varchar(50)
)

insert into #Tmp(Drive, MBFree) exec Server1.master.dbo.xp_fixeddrives
update #Tmp set ServerName = 'Server1' where ServerName is null

insert into #Tmp(Drive, MBFree) exec Server2.master.dbo.xp_fixeddrives
update #Tmp set ServerName = 'Server2' where ServerName is null

select *
from #Tmp
order by ServerName, Drive

drop table #Tmp


Result:

Drive MBFree      ServerName
----- ----------- --------------------------------------------------
C     57859       Server1
D     384636      Server1
F     82353       Server1
C     4963        Server2
E     10605       Server2


Server1 and Server2 has to be Linked Servers in the server where you are executing the above statements.