Have same problem, just upgraded to X and now it won't open pdf files anymore. Says:
Adobe Reader has encountered a problem and needs to close. We are sorry for the inconvenience.
and reports the following:
AppName: acrord32.exe AppVer: 10.1.1.33 ModName: acrord32.dll
ModVer: 10.1.1.33 Offset: 00021ac6
I'm using XP and have removed and redownloaded several times with same result (with and without anti virus on)
Adobe has a recent(2012-01-09)KB article http://kb2.adobe.com/cps/860/cpsid_86063.html entitled "Protected Mode troubleshooting". There is a similar article (2011-09-12) http://kb2.adobe.com/cps/901/cpsid_90161.html entitled Protected View troubleshooting.
I have had issues with Adobe Reader X hanging (not responding) due to a conflict between an antivirus program and the Adobe Reader X Protected Mode/Protected Viewer sandbox features.
khwajakhurram replied to my query suggesting unchecking the default Adobe Reader X Preferences >3D & Multimedia> "Enable hardware rendering for legacy video cards" setting and this solves the issues!
What is surprising is that Adobe Reader X's 3D & Multimedia settings report that the alternative preferred renderer modes are either software or DirectX9 .....on a Vista machine with DirectX11.
The video card on this machine is definitely not legacy but a recent quality Nvidia produced card. That is also the case on the other machines with the issue I have reported.
I went a step further and I have found that performance is vastly improved by changing from the default preferred renderer mode of DirectX9 to the alternative Software setting.
http://kb2.adobe.com/cps/860/cpsid_86063.html mentions that "certain configurations of anti-virus software that have not yet white-listed AcroRd32.exe" are "Unsupported configurations for Reader running in Protected Mode". Therefore I have also excluded AcroRd32.exe from virus scanning.
I am now able to use both Adobe Reader X's Protected Mode and Advanced Security without issue and performance is vastly improved. I am certain that your issues will also be solved.
notafan777, on another thread, suggested that theses problems are due to a legacy of sloppy code and this certainly seems to be the case here with these very odd default settings influencing performance to such an extent.