Sysprocesses Says: Wait Forever

I'm doing an update on a table with about 113m rows, the update-statement is fairly simple: update tab set col = null where col is not null.
The col column is mostly null.

Sysprocesses shows three rows for this statement: 1 CXPACKET (its a dual processor, 2000 box with sp3 installed), 2 PAGEIOLATCH_SH (waitresource is filled). My guess would be that the where-clause is executed in a seperate process blocking the update.

I changed the statement into update [...] set col = null; sysprocesses shows one row with PAGEIOLATCH_SH. Executing forever.

I checked other processes including those outside sqlserver but none are using the db, let alone accessing the table involved. Even restarted sqlserver to be sure there's no dead process blocking the update. Didn't help.

So I added a search condition to the where-clause, involving a clustered index in order to reduce the rowcount. The execution plan shows a 97% hit on the clustered index, but sysprocesses shows the three rows again...

So far the profiler didn't help me out either: there's a SP: CacheInsert on the update-statement... then nothing.

What should I do?