How can I deploy an ad-hoc iOS app to a user without a Mac?
Today the most common way is to use TestFlight. This does not require a computer at all for the end-user installing the app. They will simply need to download the TestFlight app from the App Store, and use that to download and install your app.

The other options you have for deploying ad-hoc iOS apps are:

1) Using Apple Configurator (Mac-only)

2) Using Xcode (Mac-only)

3) Using iTunes (requires specific version of iTunes)

4) Using OTA installation

Options 1) and 2) are Mac-only, and thus cannot be used from Windows.

Option 3) requires you to have the 12.6.4 (or older) version of iTunes that includes the App Store. You can download it here for Mac and Windows:

https://support.apple.com/en-us/HT208079

Option 4) requires you to have a SSL-enabled web server that you can distribute your app from. You'll need to create a special plist-file for you app and link to it using the itms-services protocol. You can read about it here:

https://community.scripture.software.sil.org/t/distributing-ios-ipa-apps-with-ota-over-the-air-instead-of-apple-app-store/728

NOTE: If you haven't got very specific reasons for not wanting to do so, your best option is to use TestFlight.