urgent plz reply

select
convert(varchar(20),CONVERT(date,DateTime)) +' - '+convert(varchar(20),DATEPART(HOUR,convert(datetime,DateTime)))+'Hr' As DateHr,      
round(avg(convert(float, Mem_Avail_MB)),0) "Memory Available (MB)",        
round(avg(convert(float, ADQL)),3) "Avg. Disk Queue Length",               
round(avg(convert(float, ADRQL)),3) "Avg. Disk Queue Length",                
round(avg(convert(float, ADWQL)),3) "Avg. Disk Queue Length",                
round(avg(convert(float, Proc_time)),0) "%Total Processor time",        
round(avg(convert(float, BufferCacheHitRatio)),0) "Buffer Cache Hit Ratio",        
round(avg(convert(float, TargetServerMemoryKB)),0) "Target Server Memory (KB)",        
round(avg(convert(float, TotalServerMemoryKB)),0) "Total Server Memory (KB)"        
from dbo.Tbl_Perf_Monitoring
-- where CONVERT(date,DateTime)='2012-12-12'   
group by (CONVERT(date,DateTime)), DATEPART(HOUR,convert(datetime,DateTime))       
order by (CONVERT(date,DateTime)), DATEPART(HOUR,convert(datetime,DateTime))

i am getting following error

Msg 241, Level 16, State 1, Line 1
Conversion failed when converting date and/or time from character string.
wt is this