Not trying to be a jerk here, I am not even sure this is possible. Thought I had done the same thing, but my issue was the browser crashed before the process was complete. Cisco support has told me that if you try to apply the firmware to a non-supported device it will simply fail "without" an attempt to apply. The issue here is that you need to be patient and leave the browser open for some time while the update is applying (with RV042G). If you close the browser early, the update "will" fail and may brick the router (at least this is what I was told).
With that said, your only recourse is what Tom has suggested. Here is a link to the firmware rescue utility which may help as well: http://www.cisco.com/cisco/software/release.html?mdfid=282414010&release=4.0.0.7&relind=AVAILABLE&softwareid=282465800&rellifecycle=&reltype=latest
Hope this helps.
John
Update: Looks like the restore utility I referenced will not work for you. You will have to use TFTP. The tftp command is not available by default in Windows 7 but can be enabled by turning on the TFTP Client Windows feature from Programs and Features in Control Panel. Here is how: http://avtech.com/Support/index.php?action=kb&article=108
Here are further commands: http://technet.microsoft.com/en-us/library/ff698993.aspx
