I tried several screen share systems like go-to-meeting.com, join.me,freescreensharing.com, freeconference.com and freeconferencecall.com; required the attendee to download software and/or first upgrade me to Pro version and upsell me after 30 days. Some had no call-in support and software would not support attendees with Windows XP.
Dualmon.com free screen share system does not require the attendee to download software, works on XP, no first upgrade Pro version and has live phone support.
I have tested their free screen share with friends and all say the image is great, plus I have seen the shared screen. Dualmon does have a Pro version for all of one dollar per month with great features.