You could just install the new distro on the partition(s) that hold your slax installation. One stone, two birds.
If that doesn't suit you:
To format/partition drives in Windows7 have a look at this
To restore your Master Boot Record (in case slax's boot manager is installed on it), you can use fixmbr
Btw, I'm not a Windows user, and this is not a Windows forum, I hope this helps though :-)