the second part of your query is running a cross join (highlighted in bold) is this something that you intended? as this can cause some over head because of the way that it works.
Declare @InstID int
select @InstID=S.InstID
from SRQMST S
where SRQNumber=@SRQNumber
if(@InstID<>-1)
select
S.*,I.SupplierID,I.ModelNumber,I.SerialNumber,i.InvoiceNumber,i.InvoiceDate,i.PONumber
,i.PODate,i.ExtendedWarranty,i.FreeServiceCount,i.FreeServiceWarranty,i.PEWarranty,i.SalesNoteNumber
from SRQMST S,InstrumentMST I
where S.InstID=I.InstID and SRQNumber=@SRQNumber