is there a tool to keep snr stable for vista

Depends on what you mean. Do you mean does the DMT tool work on Microsoft Vista?

i dont know ,how does that one work

I don't use it. It's a tool which allows you to manipulate target SNR margins with some routers.

See http://dmt.mhilfe.de/ (the program is German in origin).

What are you trying to achieve?

at the moment my snr is 7.7 7.8
but then it will just drop to about 4

That means there is more noise on your line. The SNR margin shows how much safety margin there is left in your line (signal) to cope with noise. When your router starts it normally has a SNR margin of 6dB which is plenty. As night falls noise tends to increase and uses up some of the margin. The change in margin throughout the day is quite normal.

SNR stability has nothing at all to do with which operating system you use it is determined by your router, your phone line, and the DSLAM at the exchange (plus external factors that can screw it up).

In fact it is completely outside of your control - you may be able to force a higher target margin using some routers but that won't stabilise the snr it will just reduce the chances of a forced resync.

ok thanks to all for your help and advice