If you'd like to get into nodal compositing on Windows ...for free... then your best options are probably:
Natron (http://natron.inria.fr/)
Blender (http://www.blender.org/features/)
Blackmagic Fusion (https://www.blackmagicdesign.com/products/fusion)
There is also Autodesk Composite, but I believe it's no longer developed.
Composite (https://apps.exchange.autodesk.com/)
If you're willing to pay $999 for Fusion Studio, that'll give you the Fusion compositing app itself, along with their Avid Connect plugin which allows you dynamically link to Fusion from Media Composer ($49/mo, http://www.avid.com/) so you can see your shots in context. Blender also has a simple timeline, but it's not built for editing or conform.
Fusion isn't "realtime" the way Flame is, but with some tweaking it can be very interactive. I posted some settings online that worked well for me:
http://www.jasonmyres.com/2014/11/fusion-7-performance-tuning/
Blender actually leans more toward animation and modeling, but does have nice nodal compositing features. Natron, is the newest of them all, and is essentially a Nuke clone (almost to a fault), so if you see yourself as a pipeline compositor, it might be an option. However, The Foundry now offers a "non-commercial" version of Nuke for free if you just want to practice and are ok with being limited to an HD project size.
http://www.thefoundry.co.uk/products/nuke/non-commercial/
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