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
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!)