Going through this archway is the only way to complete two quests (Golden Claw, and the Dragonstone). According to the map, I should be able to go through the wider one to the left of it - but that one is completely overgrown with roots.

About its location: It's awhile past the gate with the snake+snake+whale key, and past the wooden spiral steps. Somewhere further North of this archway is where the markers for both of those quests are.

I try to walk through it, and my character moves as if it were a wall. Same if I try to be sneaky and go through it while strafing sideways.

The only unusual thing I can note is that, sometimes, a box appears while in the inventory screen asking me to install a Script Extender, but this seems to be a mod that doesn't appear on the list of installed (whether active or disabled) mods. (No idea what's up with that.)

I'm playing on Steam, if that's helpful to know.

Any help or ideas are highly welcome! Thanks

Did you already encounter the Dunmer and the spider? If not it might be cobwebs blocking your way.
You can destroy them easily with a weapon (or magic or your fists, I guess).
If you don't see them it might be a problem with your graphic settings or a texture mod.
Or you have destroyed them already and it didn't trigger.
Otherwise, I have no idea.

About the script extender box: might be that you are using SkyUI, it requires the script extender for some advanced features.
Get it http://skse.silverlock.org/.

Seriously, get it. It's stable and usually worth to have. It won't so anything on its own, but a lot of mods require it.

I have not. Where is the spiderweb? I may not have gotten that far on the map. SkyUI is not installed, (neither in active or inactive mode) - right now, the active mods (according to the data files - all but the last 2 installed by my friend) are:

• 100X Carry Weight
  
• Skill Books Library
  
• Lock Picks Cheat
  
• Train 10 Skills Per Level
  
• 10K Gold Coins
  
• Headbomb's Better Sorting
  
• Unofficial Skyrim Patch

It won't let me upload the screenshots I took - of both the archway and its location on the map. (The forum says I don't have permission to post links - I uploaded the images to imgur - not sure how else to get them up here)

Could you post a part of the link? Without the imgur part, like: imgur.com/gallery/example.
Then I could link the picture for you.

qkTby.png
5C8Cq

The map is from Bleak Falls Temple, just to clarify.

http://imgur.com/qkTby and http://imgur.com/5C8Cq.
There's a spiderweb right there. Two hits should be enough to cut it.

I've tried cutting through those roots overhanging the left arch, and it does nothing. It's the open archway on the right that won't let me by. [In reference to image 5C8Cq ] To me, it looks like there's spider's eggs (with some webs on them) in the larger, rectangular room through the archway - but I'm stuck in that little vestibule before that room.

Also, installing the Script Extender did nothing.

Go to the open archway and slash (or hit it with magic) a few times, it must work.

Yes, the empty archway should have webs blocking the way.
Maybe you can't see them for some reason.
Try hitting the "air" a couple of times.

What's your GPU and which driver are you using?
Might want to do a search in the technical issues subforum for "missing" or "invisible cobweb".
Maybe others had similar issues.

This, you're normally supposed to slash your way through webs, but if they aren't appearing for you, it's obviously some kind of glitch or issue.

Aha! That worked! Thanks so much!