Update/Installation Failure

using an older configured system running XP SP3. have tried several times to download an installation/update of both Reader and Flash Player but always fails. usually unable to click on the "Install/Update" button from step 1 of 3 before it quickly disappears but if can process stops ie. step 2 screen is never reached. if this step achieved manager responds with an "encountered error" message. my only active (?) anti-virus program is McAfee. any suggestions for solution?