Windows doesn't seem to like uninstalling ToB sometimes and its a known issue.
How To Manually Uninstall BG2: Throne of Bhaal (As described at the
Sorceror's Place Troubleshooting pages)
Here is the procedure to manually uninstall BG2: Throne of Bhaal. This will also uninstall BG2: Shadows of Amn.
Note: to open Regedit go to Start - Run: type Regedit and press Enter. (Be careful editing the Registry!)
1. Edit the system registry using the regedit utility. Remove the following keys under "HKEY_LOCAL_MACHINE\SOFTWARE\ Microsoft\Windows\CurrentVersion\App Paths\":
BG2Main.exe
bg2ep202.exe
2. Remove the following keys under "HKEY_LOCAL_MACHINE\ SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\":
English: {B8C3B479-1716-11D5-968A-0050BA84F5F7}
French: {43D4C77E-4339-11D5-96E7-0050BA84F5F7}
German: {5B09F344-4406-11D5-96E8-0050BA84F5F7}
Italian: {C3817924-440B-11D5-96E8-0050BA84F5F7}
Spanish: {68F40945-449D-11D5-96E9-0050BA84F5F7}
3. Go to your Program Files directory and find the hidden directory "InstallShield Installation Information" and delete the directories labeled:
English: {8DAE4336-2B71-11D4-9A6C-006067325E47} and {B8C3B479-1716-11D5-968A-0050BA84F5F7}
French: {43D4C77E-4339-11D5-96E7-0050BA84F5F7} and {5E30BDEB-9307-11D4-9AE0-006067325E47}
German: {5B09F344-4406-11D5-96E8-0050BA84F5F7} and {5E30BDEB-9307-11D4-9AE0-006067325E47}
Italian: {C3817924-440B-11D5-96E8-0050BA84F5F7} and {5E30BDEB-9307-11D4-9AE0-006067325E47}
Spanish: {68F40945-449D-11D5-96E9-0050BA84F5F7} and {5E30BDEB-9307-11D4-9AE0-006067325E47}
4. Manually delete all the files and directories created by the previous installation attempts (you should find these in the game folder you normally install BG2 into.) and try again to install from the CD.