Lots of blocker processes are making applicaiton slow

I have one reporting application which was running smoothly for last 3 years without any problem.For last 3 days I observed lots of blocker
processes which brought down the performance
Every maintainence activity(like re indexing,update statistics,reorganizing,shrinking etc) is working fine.
There is no changes done on db server,no schema change,no new table created, sp added,deleted etc.
Other specification of the enviorment are:
Size of mdf file is 300GB on a SAN box.
Free space available is 2TB
RAID is 5.
RAM is 32 Gigs.
OS version is win 2003 64bit

SQL 2005 65bit
Lots of blocker process exists on this db and this is the reason for the slow performance of application.I cant keep killing process every second.
neither this help.I restarted the db sever and app server,but no help.
I am wondering what is the root cause of the problem and how to fix this.I cant blame the application developers who wrote sp's as these sp are running for last 4 yrs and are tuned.
I cant change the default isolation level as per business requirement

I think you need to start trobleshooting by configuring 'blocked process threshold' which triggers an event when a process is blocked for a more than a specified time. Read about this feature more in BOL. This will give you a starting point.
exec sp_configure 'show advanced options', 1;
reconfigure;
go
exec sp_configure 'blocked process threshold', 30;
reconfigure;
There are many resource available in internet .
Check these
Troubleshooting Performance Problems in SQL Server 2005
http://blogs.technet.com/rob/archive/2008/05/26/detecting-sql-server-2005-blocking.aspx
Madhu