Vlk
2
The update should work just well even if you’re not logged on as an admin.
Do you use the fast user switching feature of WinXP?
Does the non-admin account has enough rights to the temp directory? And to the avast\data directory? (assuming your hard drives are formated as NTFS; you can check this using the Security tab of the Folder Properties in Explorer).
Please tell me how proficient you are with computers so I can select the appropriate wording. 
Thanks
Vlk