Unable to cast COM object of type 'Word.ApplicationClass' to interface type 'Word._Application'"

I'm sorry it's taken so long to get back to this. I just hope it will be useful.

Let me recap what is (or may be) happening. When you tell ACT! to write a letter or do something else involving Word, it sends a request to Windows that says something like "find the executable file for Word, start it up, and pass this text to it". Windows then looks in the registry for an entry that says "when a request for Word comes in, send it to this assembly". ("Assembly" is program-speak for one kind of executable file.)

Each version of Word has an assembly and a series of registry entries that point to it. In your case, Office 2010 installed an assembly named Microsoft.Office.Interop.Word whose version number starts with 14, and Office 365 installed an assembly named Microsoft.Office.Interop.Word whose version number starts with 15. Although you've removed the version 15 assembly, there are apparently still registry entries that tell Windows to try to use it. The objective now is to get rid of them.

The first step -- and don't be tempted to skip it -- is to back up the registry so it can be restored if something goes wrong. Follow the instructions on this page in the Sage site. It also wouldn't hurt to tell Windows to create a Restore Point.

In the Registry Editor, click Computer at the top of the left pane. Click Edit > Find or press Ctrl+F. Type or paste Microsoft.Office.Interop.Word into the box, check all three boxes, and click Find Next.

The first item it finds should be in the key
   HKEY_CLASSES_ROOT\CLSID\{00020906-0000-0000-C000-000000000046}\InprocServer32
On the right side, the Assembly entry may say
   Microsoft.Office.Interop.Word, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71E9BCE111E9429C
Double-click the word Assembly to open an editing window. Change the 15.0.0.0 to 14.0.0.0 and press Enter.

Press F3 and the next find will be the 14.0.0.0 key just below the InprocServer32 key; this is OK, so leave it alone.

Press F3 once more, and it should find the Assembly with version 15 in the 15.0.0.0 key just below the 14.0.0.0 key. That is not OK. Click the 15.0.0.0 in the left side and press Delete. A message will ask whether you want to permanently delete the key. Double-check that you have selected the 15.0.0.0 key, and then click Yes.

Press F3 again, and you'll find the same arrangement under the key
   HKEY_CLASSES_ROOT\CLSID\{000209F0-0000-0000-C000-000000000046}\InprocServer32
(notice that the difference from the previous key is only the part I've marked in bold). Repeat the same steps as before.

Repeat the whole sequence for the keys that start with {000209F1-, {000209F2-, {000209F4-, {000209F5-, and {000209FF-.

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Next, locate the key HKEY_CLASSES_ROOT\Installer\Assemblies\Global. Unfortunately, the Registry Editor doesn't have a GoTo command; you need to look through the alphabetized list at each level and drill down. (There are third-party registry editors that make this a lot easier.) On the right side, locate and delete the entry that starts with Microsoft.Office.Interop.Word,version="15. Also locate and delete the one that starts with Policy.11.0.Microsoft.Office.Interop.Word,fileVersion="15.

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Press Ctrl+F and modify the search term to look for
   Microsoft.Office.Interop.Word, Version=15
(notice one space between the comma and the V). That should find the key
   HKEY_CLASSES_ROOT\Record\{002686EA-BC17-3B7C-BE4E-ECA572100016}\15.0.0.0
Delete that key. Continue searching with F3 through the Record key and delete each 15.0.0.0 key. There are probably a couple of dozen of them, one for each feature of Word as indicated by the Class entry on the right.

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Locate the key
   HKEY_CLASSES_ROOT\TypeLib\{00020905-0000-0000-C000-000000000046}\8.6
in which the right-side entry PrimaryInteropAssemblyName includes the Version=15.0.0.0. Delete the 8.6 key.

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If you're running a 64-bit version of Windows, repeat everything that's gone before for the identical series of entries under HKEY_CLASSES_ROOT\Wow6432Node\CLSID.

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Reboot and see whether ACT! can do what it should. If not, or if something else has gone wrong, double-click the .reg file created by the backup procedure to put the registry back the way it was.

In the Windows Control Panel, go to Programs & Features. Select the Office 2010 icon and click Change. On the first page of the wizard, click Repair and then Continue. Let it finish.