I have taken backup of http://mysite:300/ site and restored it in some other server say http://newserver:300/. when I have created subsite using template already exist, site creation is successfull but all the webparts got disturbed so am not able to view the page. what may be the cause or is there any other way I can do this??
If the webparts are custom webparts then you need to make sure they are installed and deployed to the new farm. Check out solutions or other applications that you deployed on the original farm and find the one that contains the web parts. The assembly and class of a particular web part you can find in the site collection web part gallery.
The pages only contain references to the type of web-part and the configurable properties set for this instance; the web part implementation class must be installed.
What version of sharepoint are you using?
I've experienced this issue where it will use the same GUID's for the lists, even though they are not in the current context/are duplicates. Try copying a site and see if the problem is the same. If so, avoid copying webparts until you upgrade to 2010 (assuming your on 03/07).
To fix the issue for the pages/sites you've already made try going into designer and deleting/modifying the webparts. I don't believe simply changing the GUID will work as it needs to actually be generated by creating a new webpart.