"It sounds like the 64-bit version didn't get installed. According to Avast's webpage:
Support for 64-bit Windows
avast! Home/Professional now fully supports the 64-bit Windows platform. ALWIL Software is anticipating massive take-up of this platform, as Windows XP 64-Bit Edition will support up to 32 GB of RAM and 16 TB of virtual memory, enabling applications to run faster when working with large data sets. Applications can preload substantially more data into virtual memory, allowing rapid access by the 64-bit extensions of the processor. This reduces the time for loading data into virtual memory or seeking, reading, and writing to data storage devices, making applications run faster and more efficiently.
Regular (32-bit) antivirus applications are unable to operate correctly on the 64-bit Windows platform because they rely on 32-bit kernel-mode drivers. The new avast! uses native 64-bit drivers, delivering the same level of protection as in the 32-bit Windows environments. The installation package is the same for 32-bit and 64-bit versions - the setup program automatically detects the operating system it is running on, and installs all necessary files. You may find more information on avast! Antivirus and Windows x64 Edition page.
Note the sentence I emphasized - since you're running 4.8, which is the latest version, I would try uninstalling and reinstalling Avast. I used Avast 32-bit on Windows 7 until I found Microsoft Security Essentials (which I can confirm works perfectly with Windows 7 x64 and is free to boot!)"