Query Runs On Sql7.0 But Keeps Running On Sql2000 Without Output

The following query runs fine on sql 7.0 but it kind of hangs/keeps running without any output on sql server 2000
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Set @cmd = 'Update ABCD Set
'+@day+'_LB = IsNull(LB,0),
'+@day+'_UT = IsNull(UNIT,0)
From tempData as T
Where T.STORE_NUM = ABCD.STORE_NUM And T.ITM_ID = ABCD.UPC'
execute (@cmd)
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But if we hard code the @day parameter and run the query like this it runs very fast on sql 2000....

Update ABCD Set THIRD_LB = IsNull(LB,0),
THIRD_UT = IsNull(UNIT,0)
From tempData as T
Where T.STORE_NUM = ABCD.STORE_NUM And T.ITM_ID = ABCD.UPC
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