In the program options though, if you put it on the most secure options, (Wash slack space, Add bleach, and set on Guttman with 35 passes) you should be okay from all but the most motivated, well-funded intelligence analyst looking to uncover some major international intrigue from your hard drive.  Unless you have reason to think such a person would suspect you of that...

It's probably as good as any washer program,  which work the very same way.  

You might take other measures if you're really concerned, like encryption.  I suggest PGP-- which has it's own disk-wipe feature too that's far more versatile.  You could set it to work wiping the unused space on your disk for a full night.  You get it from pgpi.org. The encryption feature is also good, but as I tell everyone, encryption has an alpine learning curve.  

Window Washer should be fine.
Most "shredders" that do a 7 pass (or more) destroy data to the extent that most software recovery won't be able to get the files.

Now, if you're talking about the FBI looking on the drive then no, Window Washer probably isn't enough, but the equipment for that kind of recovery costs over $100,000 and companies that have it charge upwards of $800 an hour or more to do the recovery.

So, you probably don't need to worry about it, and  Window Washer should be enough for you.