You're using add-ins with Outlook, Excel, or other Office apps. While 32-bit applications can work with add-ins, they can use up a system's available virtual address space. With 64-bit apps, you have up to 128 TB of virtual address space which the app and any add-ins running the same process can share. With 32-bit apps, you might get as little as 2 GB of virtual address space which in many cases isn't enough and can cause the app to stop responding or crash.