When I try to download 5.1.864 version from Avast’s site the Cnet download site offers me the old 5.0.677 version. Why I can’t download the new version? Is the stable 5.1.864 even out yet?
Ok. Thanks for both of you. 
It is recommended that you do a clean uninstall using http://www.avast.com/eng/avast-uninstall-utility.html
Re-boot, then install 5.1.874. Hope this helps.
nope >>> upgrade or clean install using the normal uninstaller first will do.
I tried to use unintsall/repair before installing 5.1.874 and it didn’t work. I ended up using the avast! uninstall tool and re-boot. Then I installed the new version.
To answer the OP’s question: The reason is that v.874 is a beta version for those having problems with the pre-release version (v.864), and this version is available on the forum page, as you discovered.
The proper way to uninstall and do a clean install of Avast is as follows:
Save a copy of newest version of Avast (Beta build 5.1.874) for the product you need and save it to your HDD:
Free: http://files.avast.com/files/beta/5.0.874/setup_av_free.exe
Pro: http://files.avast.com/files/beta/5.0.874/setup_av_pro.exe
AIS: http://files.avast.com/files/beta/5.0.874/setup_ais.exe
2. Download the Avast Uninstall Utility http://www.avast.com/uninstall-utility and save it to your HDD (it has uninstall tools for all Avast versions and products).
3. Disconnect from the Internet at this time.
4. Go to Control Panel and uninstall Avast through Add/Remove Programs if possible and reboot.
5. If Step 4 fails, boot into Safe Mode (hit F8 repeatedly) and run the Avast Uninstall Tool.
6. Reboot.
7. Install the newest version of Avast and reboot.
8. Get Internet access and register your copy or add the license key for Free, Pro, or AIS.
9. Update the Avast definitions.
Please let us know if this fixes your problems. Thank you.
Well, I’m a bit confused. :o Is the v.864 still a beta or is it a stable version?
public build: 5.1.864
available betas: 5.1.874 and now [b]5.1.884 /b
Ok. Thanks for the info!