Canadian winters, I can expect temperatures as low as -30C. Though it's been pretty warm lately.

Here's my big problem. With underwear (boxers), long jonh's and my pair of waterproof marmot pants, the fabric of the underwear and the long johns tend to clump btween my skin and the seat, which create incredible chafing, sometimes to the point of opening a hole in the skin. Hurts!!!!

What can I wear to prevent this, that will keep me warm enuf for the winter here...

lycra bike shorts with no underwear + long tights + long johns (you might be able to lose these, too if you have the long tights) + waterproof pants

your chafing problem will go away, and you'll be dry and toasty !

start with a good pair of cycling shorts or tights. then layer on top of that. the layer that comes in contact with your skin should be tight enough not to bunch up when you pedal.

I put my long knickers on first (no underwear) everything on top.

Never had a problem...and yes, I wash my knickers often.

I ride to about -15C, found that bike shorts under lined nylon pants to be warm enough for most rides. Ben, you might want to try polyester or polypropylene long johns and boxers under yours - If what your wearing is cotton, the wet doesn't get wicked away from your skin, whereas the poly wicks the moisture away, makes everything a lot more comfortable.

Thanks for the answers all. I will look for cycling tights and layer above it.

I wasn't too keen on the idea of cycling tights, but I guess I have to now.

I wear skivvies in the winter under my Performance Triflex Bibs.
It feels good.
I wear some baggy athletic shorts over my tights for either style or modesty reasons - I guess style.

Bike shorts, mid weight Patagucci capilene longjohns, REI windfront tights. Good to below zero F. (-18 C or so) Throw a pair of fleece pants on top of those and you're good down to -20F (-29C) I don't ride if it's more n' -20F. So call me a wuss. Too many bad things can happen really quickly at that temp.

Adventure is just poor planning- Inuit saying.

Ben, im not sure what kind of money you have, but the gear that others have listed for you can be very expensive (ie padded cycling shorts, patagonia capilene stuff)
ive been wearing (anybrand) polyester polarfleece shorts/pants as a base layer with great results for years in every season! The reason you are chafing is because youre wearing cotton in the wrong place, your boxers are in a bunch, and it sounds painful.
Cycling tights/shorts are good, but they arent for me.
if youre a frugal baggy pants cyclist (like i am) you have to try fleece as a base layer.
good luck and fast healing.