Removing/deleting the virus would certainlu change the hash of the file. One thing we always do when encountering a virus on a evidence disk is to try and ascertain what it is that virus actually does.
Then we look for digitial evidence that might indicate of the virus actually went "active".
Regarding the hashing, you might have a look at a new tool written by Jesse Kornblum --> w w w.dfrws.org/2006/proceedings/12-Kornblum-pres.pdf
His tool creates and checks hashes on parts of files.
Never ever intentionaly modify the evidence. You should be working on an exact copy of the evidence. Note in you case notes that you found the virus, attempt to determine the exact location of the virus (file slack, unallocated space, zip archive, jpeg, etc.). Some viruses and malware can infect your workstation. You need to obtain as much information as possible to identify and prevent any secondary infestation. Insure you document this fully. Don't devote your life to it - because this may have nothing to do with your case! However, you still need to protect yourself and others.
For example, you are working a case and must provide Discovery information. You provide counsel with the requested information and forget about the virus. (Now saying that anyone would -- just suppose) Counsel's equipment becomes infected. How valuable is your work and testimony now? That is just one of the hundreds of questions and problems that will now have to be resolved. Hopefully, you or the firm you work for has good insurance!