“Where can I get free loops and samples for making music in Windows?”
One thing to look for in acquiring loops is making sure they are royalty free, whether they are free of charge or not. Royalty free music typically either has a one-time charge or no-charge, allowing you to use it as you see fit without paying the creator each time you use or perform a portion of their creation.
KVR Audio emerged many years ago as an excellent source of drumkits, samples, loops, sound banks and other tools for creating your own music. Many of the downloads are free and free of any complicated rights for commercial use.
Freesound Project is a collaborative collection of sounds and loops released under a variety of Creative Commons licenses. You have to check each track to see whether they are available for commercial use or not, but all tracks are available for non-commercial use.
BeatSuite offers a bunch of free royalty free tracks. The BeatSuite offerings come with a couple of caveats. The tracks are free for non-commercial use. The tracks tend to be lower quality than pay versions of the same loops. Registration is required in order to access free tracks.
Meanrabbit offers a bunch of great sounds you can loop or use to build loops. Many of the sounds included are part of the urban landscape, from trains to telephones, but there are also some fairly unique sounds available too.
If you really want quality loops, there are many collections available at reasonable rates. Free sounds will get you started, but most of them sound inferior to pay versions. An additional option is to build your own loops using sounds included in applications list Mixcraft.