I have downloaded the free version - but now I have received an error message which states that I do not have sufficient user rights, for any upgrades. I have even attempted to uninstall the software to then reinstall. Any ideas folks. Many thanks.
Not knowing what OS you are using try…control panel…programs and features…add/remove (XP) uninstall (Vista/Win7)…double click avast…scroll to repair and click…re-boot. Hope this helps. 
Are you installing from Admin. account? You should be, if not.
edit: btw, your email is showing public on the forum
I have exactly the same problem. Win 7 64 bit. I uninstalled the prior version of Avast free using avast clear. Have downloaded the install file three times. I am an admin - “not enough rights…” Tried “Run as Admin…” Same exact thing. This all started after a security update from Microsoft last week or so. I have used Avast free for years - but now, I cannot install. Here is full “details” from error message I blocked some parts of the path for security using xxxx…:
12.07.2011 18:06:49 general: Started: 12.07.2011, 18:06:49
12.07.2011 18:06:49 general: Running setup_ais-4b3 (1203)
12.07.2011 18:06:49 system: Operating system: Windows 7 ver 6.1, build 7601, sp 1.0 [Service Pack 1] x64
12.07.2011 18:06:49 system: Memory: 40% load. Phys:3708048/4194303K free, Page:4194303/4194303K free, Virt:2025980/2097024K free
12.07.2011 18:06:49 system: Computer WinName: XXXXXXXXXX
12.07.2011 18:06:49 system: Windows Net User: XXXXXXXXXXXXXX
12.07.2011 18:06:49 general: Cmdline: /sfx /sfxstorage “C:\Users\xxxxx\AppData\Local\Temp_av_sfx.tm~a01112” /brandcode “A” /srcpath “C:\Users\xxxxx\DOWNLO~1” /sfxname “setup_av_free”
12.07.2011 18:06:49 general: DldSrc set to sfx
12.07.2011 18:06:49 general: Old version: 465 (1125)
12.07.2011 18:06:49 general: Install check: ‘C:\Program Files\Alwil Software\Avast5\AvastUI.exe’ does NOT exist
12.07.2011 18:06:49 general: SGW32AIS::CheckIfInstalled set m_bAlreadyInstalled to 0
12.07.2011 18:06:49 registry: ERROR!:Cannot create registry HKLM\Software\AVAST Software\TestSetup, error code: 0x00000005
12.07.2011 18:06:49 general: Error:Not enough user rights to continue.