Stuck with a 64-bit Windows 7 on a 32-bit machine

I have a Lenovo Thinkpad laptop, SL500, 2GB Ram (DDR2). Several days ago, I went to a repairman (for lack of a better word) to upgrade my Ram to 4GB. After he plugged in 4GB Ram, the repairman installed the 64-bit version of Windows 7 (I need it for some specific reasons), but it seems like he screwed up and was unable to boot Win 7 (it justs stops at the WIndows icon and "starting Windows"). Then, after all options have been tried, I decided to plug back the 2GB ram and go back to the way it was, but the problem is the OS is now 64 bit, while the RAM is 32 bit. Now I'm unable to do anything. When I try to turn on, it just stops at the Windows icon. When I try to do a safe mode, it simply stops at "Loading ...... ahcix64s.sys" . Please help me, thank you very much

Update: I was also unable to load the last known good configuration , since this is the only thing I see:

Windows Boot Manager  

Windows failed to start. A recent hardware or software change might be the cause.  
To fix the problem:

  1. Insert your Windows installation disc and restart your computer. 
  2. Choose your language settings, and then click "Next."
  3. Click "repair your computer."

If you don't have this disc, contact your system administrator or computer
manufacturer for assistance. 

  File: \Boot\BCD

  Status: 0xc000000f

  Info: An error occurred while trying to read the boot configuration data.