Right click your computer icon go to properties -> advanced system settings and then under the advanced tab go to system properties.
Once there on the advanced tab under performance, click settings then go to the data execution prevention tab and add Skyrim to the exception list.
For better investigation, you should know your Exception code. !!!
OR
Start -> Search: type cmd. Right click on search results ‘cmd’ and click on "Run as Administrator".
Then write this down:
bcdedit.exe /set {current} nx AlwaysOff (You should get a successful confirmation)
Reboot...
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If that not helps you out, start windows in clean boot mode: http://support.turbine.com/link/portal/24001/24001/Article/88/How-do-I-perform-a-Clean-Boot-of-Windows
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Obviously you are using some kind of tool for having 500 mods installed. (and obviously you have merged a lot of Esp's)
If a merge giving you a ctd, it usually means that one or more of the files you have merged doesn't like being merged.
Another possibility is, if you did not check for errors, you may have merged mods with corrupted data. !!!