If it's pirated, you can't really do anything. The guy that cracked it probably ****ed up. Or you could try re-installing, but backing up your saves. If it's a legit copy, I was looking around and found this, here:
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The OP there could run other games quite well maxed out, but for Skyrim it was lagging pretty bad. Here's what worked for him:
Originally posted by another user
I've found that 11.11 performs poorly for my gf's Radeon so I downgraded to 11.10 and now it runs great. There must be something else going on though, because your system should be able to run this on high with AA and AF turned on.
try these and see if they help.
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or
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My girlfriend runs the game at 1920x1080, everything on high, except blur on medium , decals ultra 2XAA, 8XAF and gets 30+ outdoors, a lot higher indoors.
her specs: AMD Athlon II Regor 2.7ghz, AMD Radeon 5670 1 gig, 3 gig DDR2(it's actually 4 gigs but Windows XP only uses 3.2 gigs of it.).
Oh, and try this too on top of the above files. This helped me quite a bit as well.
Large address aware:
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I'd recommend reading the full post