ok first it's better that your SD card is clean before starting,then you will need Android Loader,it's just a tool that will allow you to chose between the two OSs on boot up you can get it
here and install it to system (not to the SD) now you need to chose a ROM you can use
this oneit's a vanilla android 2.3.7,now unzip the file,everything should be in a folder called "Android" (without quotes)now i don't know if the link that i have gave to you have a "data.img" file (it determines the internal storage)if it does have just copy it inside the Android folder if it does not you can install android and it will make one by itself (but could took up to 30 min to boot up the first time) now copy the Android folder to the root of your SD card,now make sure your phone is charged and in WinMo file explorer access the android folder run CLRCAD (nothing will show up) then run HARET you should now see a small pop up and some text line on the screen BE PATIENT,when android finishes starting you will see a splashscreen and at the end and android lockscreen (that's mean you are done)
now am gonna give you few tips to be on the safe side before flashing any new rom (using the same way) it's better to format the SD card,and it's always better even if it is slower to let the Android ROM write it's own data.img since if you downloaded a data.img file from the internet maybe it will not work with your ROM so it's always better to let android write this file on first boot,always backup the data.img to your PC.
well this is all for now am ready for any future help :)