select u.userID, u.nameOnline, u.lastLoggedIn, u.emailAddress, 
       isnull(a.IMCount, 0) as IMCount, 
       isnull(b.MailCount, 0) as MailCount
from   tblUserDetails u
       left join
       (
           select UD.userID, count(*) as IMCount
           from   tblUserDetails UD inner join tblInstantMessage IM
                  on  IM.messageToID = UD.userID
                  and IM.checked     = 0
                  and IM.Date        < UD.lastLoggedIN
                  and IM.Date        > dateadd(day, -1, getdate())
           group by UD.userID
           having count(*) > 0
       ) a
       on u.userID = a.userID
       left join
       (
           select UD.userID, count(*) as MailCount
           from   tblUserDetails UD inner join tblMessage M
                  on  M.messageTo    = UD.userID
                  and M.checked     = 0
                  and M.Date        < UD.lastLoggedIN
                  and M.Date        > dateadd(day, -1, getdate())
                  group by UD.userID
           having count(*) > 0
       ) b
       on a.userID = b.userID
where a.IMCount is not null
or    b.MailCount is not null