I think that first of all you have to unlock your phone with Unrevoked. Also, if you have froyo 2.2 there is no problem at all. Just DL latest Unrevoked (I suggest Linux version) , put your phone in Hboot and voila, after couple of minutes it is unlocked. Problem (I think) exists with newer version 2.3 where also Boot is changed to version 1.0 (old was 0.9xx).
to check this, turn off phone and then on, holding pushed POWER+VOL DOWN buttons.
First at all, re-format your card (do a backup first ) . One FAT32 partition and one EXT3 (EXT4) but no larger than 1GB . I recomend P*aragon partitioning tool or P*artition manager. Also, with P*aragon alignment tool align FAT32 partition.
After installing Clockwork mode to your phone, just follow instructions from MIUI web site , or just simply:
- make USB mount to SD card though Hboot (recovery mode).
- Put on root of your SD card zip file of new MIUI.
- Put language pack, too, if needed.
- Put radio package if needed (can be found on htc desire user guide on miui site - link)
Return to previous menus.
Do a dalvik, cache and system cleaning.
Install from zip from SD card: miui sistem file, radio, language pack.
reboot and enjoy.
Addition: Be carefull: When creating partitions, both (FAT32 and Ext3) must be Primary partitions and no drive letters (none).