csrss is a legitamite file that Windows needs to operate properly. If you are able to get passed this error message and everything is working ok with Windows, then perhaps you have been infected with the nimda.e virus. This virus also creates a file named csrss, but of course you do not want this one on your computer. The way to tell them apart is by thier location on your computer. Depending upon what anti-virus software you are running, it is also possible that this virus has been removed, that is to say that the rogue csrss file has been removed, but the rogue program is still tying to start up and is looking for csrss and not able to find it.
One idea is to try this free nimda.e removal tool found HERE