Anyone who has ever had to recover "permanently" deleted files from a computer would know that a simple fdisk, format, and re-install is not enough.
I agree with jhoegl.... if the data is as personally private as the OP portrays it to be, it should never have even touched their company computer. Most company's have acceptable use policy's that you have to agree too before you can login, and in that policy (if you have one that is) it mostly likely states that any data you put on the computer is now property of the company that owns the asset.
The data should never have been there in the first place.
But to answer the question..... the best way is just to go through everything and use something like Spybots Secure Shredder.
I don't know how reliable SpyBot's Secure Shredder utility is but i would start there. You want a utility that will rewrite zeros to the blocks your data used; not just mark those blocks as available on the allocation table.
I wouldn't dban it though because then you could potentially get yourself in some legal hot water.