u mean Windows Live Messenger + MSN Messenger ? 

my pc cant do that, overwriting each other...

In contrast

1. Install MSN 7.5

2. Copy "C:\Program Files\MSN Messenger" to somewhere else

3. Install Windows Live Messenger

4. You can use both now but not at the same time (tested)

1st step : Install MSN 7.5
2nd step : Go into ur msn 7.5 folder Copy all files in the folder
3rd step : create a new folder( watever name la) then paste all the file that u hav copied                into the folder that u juz created( whenever u wanna open MSN7.5 after installed WLM...juz go into the folder and click the msn7.5 icon)
4th step : Install WLM

thats the way...use Windows Live Messenger when u ar doing nth other than chatting..use MSN 7.5 when u ar going to play games as it uses so much of memory    hope this will help u all ^^

PS: if u lazy to install MSN7.5 again since now u ar having WLM..kindly PM me with ur email..i will send u the MSN7.5 zip file ^^