I found that the first time I wanted to install a STOCK ICS leak an a Gingerbread phone I had to unlock the bootloader BEFORE placing the zip file (renamed to "PH98IMG.zip") on the root of the SD card. Do not unzip the file.
To unlock, go to htcdev.com, register/sign in, go to unlock bootloader and follow the clear instructions, but meticulous steps. Once unlocked, leave the window on your computer open, as you will need to re-lock after the zip file is loaded. This is not rooting, just unlocking.
Use Astro, Solid Explorer, or another file manager to be sure the extension in the file name ends in zip and not zip.zip (Win 7 tends to add a second ".zip" automatically).
You will need to wipe data and flashing will wipe as well. Apps were wiped. Photos and music did not erase on my phone, but back up in case they get wiped on yours.
When the phone connected to the computer, you will see 3 drives for your phone: a VZ logo CDROM [but it's not a CD drive]; HTC Storage; and Removable Disk. Place the zip file on the root of Removable Disk. You could re-lock now.
Then follow the instructions for flashing - pull the battery, restart holding vol down, etc. The phone should see and open the zip file. If it didn't, unlock, flash, then re-lock.
The next time you install a stock leak, you should be able to place it on the SD card and flash without unlocking.