Now you got to have the original box in which there is the imei written. I don't take any credit for this guide as i didn't write it but i found it somewhere here but it doesn't exist anymore. I'm not responsible for any damage may happen to your phone. I did it several times after flashing official 2.3.5.
It has worked fine with my IDEOS X5 U8800.
a. Drivers for U800 32bit win 7 (tested and work )
b. Drivers for Android 32 and 64 bit win 7 (not tested )
c. Software for restore
2. Enable debugging by USB in the settings.
In call type * # * # 2846579 # * # * http://postimage.org/image/tnj4s2dyj/
3. Go to ProjectMenu -> Background settings -> Usb port settings put the Manufacture mode and corfim.
4. Go again to ProjectMenu -> Background settings -> Qualcomm backround open settings put the background open. Password 0 and corfim.
5. Go again to ProjectMenu -> Background settings -> Log setting -> Log switch put log on and corfim.
6. Now conect your phone to pc and install the drivers.
You must see this at your device manager http://postimage.org/image/5lrm61agd/
7. Instal DFS and run.
8. Click Ports, select the port with the signature DBAdapter Reserved Interface (double click) to close.
9. In the DM select Qualcomm.
10) Press the SPC, the bottom will Device Unlocked.
11) Select Programming and in the IMEI write yours and press Write.
13) Check Imei, may appear after a reboot, if not try a few more times to try to set the port speed to 115200 (box to the left of DM)
Check IMEI *#06#
As soon as the IMEI restored change:
* # * # 2846579 # * # * -> ProjectMenu -> Background settings -> Usb port settings to put on the Manufacture mode google mode (or normal if you do not get up google)
* # * # 2846579 # * # * -> ProjectMenu - > Background settings -> Qualcomm backround open settings put close