Getting Windows Live Mail won't harm your computer in any way.
Windows Live Mail is by far much much better then Windows Mail. It has several functions you won't find in Windows Mail, such as RSS Feed reader and integrated Windows Live contacts, which syncs automatically with your Windows Live account online and Windows Live Messenger contacts and many more. User interface is much better and there are lots of enhancements for e-mail composing.
If you're much into mail and RSS feeds give Windows Live Mail a try. If you don't like it you can always uninstall.
The main differences is it supports Pop3, IMAP, HTTP website (EX: Hotmail) email, and is a lot more stable. Plus, WLM will minimize to a tray bar icon and has a separate Inbox for each email account. WLM will automatically import Windows Mail messages, accounts, and settings into it. Contacts will have to be manually imported from within WLM. (Tools, Contacts, File, Import, Address book for current Windows User.) To see the Menu bar, press ALT+M and check "Show all menus".
It will not hurt anything to install it. I would give it a try and see a how you like it. You can use it instead of Windows Mail or use both of them. If you do not like WLM, you can always just uninstall it.
Sure! you can try it for free!... and always be able to go-back to your "Windows mail"... by resigning all the messages you get by "Windows LIVE Mail"... since it has NOT the option to export messages in any way to match "Windows Mail" for that! it´s a shame!!