Here is another guide explaining how to flash the N900 (assuming that you are using Windows)

I prefer this guide, because of the steps which are easy to follow and the screen-shots make things a bit clearer. Follow both parts of the guide, and if you do it correctly then all your data will be gone and your phone will be like new Have fun and all the best.

P.S.: If I remember correctly, these steps only work on a 32-bit Windows OS (the last time I used them was like 5moths ago). It might be worthwhile to just try it.