You may want to either re-download php or check one of the subdirectories for php5isapi.dll.
It could be a possibility you downloaded php5 for apache.
I'm pretty sure php5isapi.dll for IIS5 or IIS6 is required. It is the API interface for IIS. If you can't find it in a distro, google it and download it.
99.99% of the tough configuration of OS ticket on a new windows server/machine will be PHP/MYSQL.
Once you get those in place, OSticket will be a cake walk.
More things off the top of my head with windows installs.
You'll need to set environment variables in windows.
Right click on "My computer" go to properties. Go to the advanced tab.
Click "Environment Variables"
The second box below will have "System Variables"
Click on Path and hit edit:
add C:\PHP (if that IS your php directory, if its not.... add the directory where your php files are, all these entries have semi-colons as seperators).
Hit ok to close "Path".
Do new and add PHPRC and its value should be your PHP directory.
You will need to reboot (pretty sure).
Also, if you have issues where you can get to the php website but theres no CSS styles (everything looks generic and out of place, almost like craigslist)
It's most likely a permissions issues.
Your windows directory that will be for hosting your webpage will most likely be C:\inetpub\wwwroot
You'll spend a good bit of time in there.
Sometimes certain fixes will state "Change permissions to 777 or 644"
You can use online guides such as wiki to tell you what exactly they mean by "777" and how the file permissions should be set.
How to set those in windows is by going to the folder in C:\inetpub\wwwroot, right clicking -> properties.
You'll see the "Security" tab.
IIS allows access to your c:\inetpub\wwwroot folder via http through a user called "Internet Guest Account" in parenthesis you will see (\IUSER_)
Sometimes you'll be requested to give write access to a tmp folder or remove write permissions from a directory and its subdirectories to secure your website.
Hopefully, that should help you get started.