To answer this question, you need to understand the difference between viruses and Trojan horses. Typically, a virus adds (appends its code) itself to an infected file so it incorporates its own code and the virus's code. Together they represent a virus-infected file. Most of these files can be cured by the Dr.Web anti-virus. Here we speak about curing files of viruses rather then curing viruses.
A Trojan horse is a complete malicious program. It doesn't inject its code into files but operates as a separate program, that's why it can't be cured but removed. Some Trojan programs compromise various system objects such as the Windows Registry. In this case we can speak about curing the system (but not the Trojan horse), which includes removing the Trojan and restoring the compromised objects to their healthy state.