if this company is in the UK I might be available for consultancy

I feel for you and really the best I can suggest is you get hold of Inside SQL Server 2000 or the sql 2000 admin guide form microsoft - this will take you through the basic stuff.

slow down is often down to stats and index rebuilds - but if your dba's were any good they should have put maint plans in place and documented the processes etc. etc.

Never assume the application is ok - experience tells me that most problems come from applications, not the server - I think the accepted figure is 20/80 hardware/software for tuning.

Or get on a sql 2000 admin course - could be tricky now but you'll probably get the best kick start. I'd suggest you get a contract DBA to assit you - to instruct a non dba will be tricky.

check out these in BOL, sp_updatestats, dbcc updateuage, dbcc dbreindex, dbcc showcontig, sp_autostats.

make sure your database is making log backups ( and full backups ) and make sure auto create and auto update stats are enabled.