Open the "Securom" folder. Don't be too surprised if you see...nothing. There are files in there that have been installed by Securom, but because they have intentionally been made to contain character errors, Windows explorer cannot recognize them and so there is no way to delete them.
If that's your only issue, you should easily be able to use the cmd tools to delete them... (cd C:\Documents and Settings\[whatever]\Securom\; dir; del [x]). It'd probably stop the game from working, though...

Did you use Micosoft's RootkitRevealer? It has been well-documented from Bioshock that a registry entry is created by Securom v7 that is identified as a rootkit. It may not be a real rootkit, but RootkitRevealer lists it as such for a good reason: it resembles a rootkit and has some of its properties.
Have a quick read of the sysinternals page on rootkitrevealer; by no means does a hit mean that it's a rootkit...