not sure if its this simple or not but i think if your using windows 7 then create a shortcut to GUIminer.exe and drag and drop it into the file "startup" in the start menu
to set your miner to mine at startup go into your GUIminer folder and open guiminer.log and set the miners you want to run to "true"
shoould look a bit like this
2013-04-18 12:41:10: Loaded: {"profiles": [{"username": "vm1990@ATI7750", "balance_auth_token": "", "name": "BTCMine 1", "hostname": "btcmine.com", "device": 0, "affinity_mask": 0, "flags": "-v -f10 -w256", "autostart": false, "external_path": "", "password": "***", "port": "8332"}, {"username": "vm1990@AMDx4", "balance_auth_token": ""
set this part to true to autostart miner "autostart": false
many thanks for the info.