first you must set the lock of the table as "row level".
then u can check the master..syslocks for whether the row of the table is locked, if already locked,u can only use kill to "release" the lock and the locking transcation will be killed too(It's a not good idea to do this, because who knows whether the transcation to be killed is active in doing the right thing?)
And u can use a simple update to lock this row.