First point is if you logout the login screen has the username 'TCAdmin' already filled in, when it should be 'administrator'. TCAdmin is the full name of (I think) the default version 5 admin user. Changing that to administrator and then using the password I thought it was solved my issue.
For resetting...
In case it helps someone else on Windows XP on version 5 of TeamCity, my .BuildServer config info was also under my current logged in user's documents and settings folder. Also I was tripped up by a space in the list of jar files in Sebastien's good answer above.
So I changed to this directory in a command prompt:
c:\teamcity\webapps\ROOT\WEB-INF\lib
and then this command line (to set password: Password1) worked for me:
C:\TeamCity\webapps\ROOT\WEB-INF\lib>..\..\..\..\jre\bin\java.exe -cp server.jar;commonapi.jar;commons-codec-1.3.jar;util.jar;hsqldb.jar ChangePassword administrator Password1
Which gave output:
Using TeamCity configuration directory path: C:/Documents and Settings/tamw/.BuildServer
Password changed successfuly