I am looking to start a java web applet, as I need something that doesn't need downloaded (saved to computer) and for the most part will work for Windows and OSX.
I have never done anything with java, so my question is can you do everything you can do with a normal java jar file on the web?
More specifically, can you write a web applet that detect window titles (Like the title of an active window), running processes, look in the windows registry, or find hard drive serial numbers?
Yes, but your applets must be trusted by user (user change some browser security settings)
The applet needs special permissions for these type of actions. It needs to be signed and trusted by the user.
Have you considered using Java Web start? If the communication with the browser from which the applet is run is crucial, then applets are probably the way to go. Otherwise Java Web start is preferred as you'll face less compatibility problems with browsers' Java plugins and different JVM versions.