virus infected?

when i turn on to start pc, my default homepage has been changed to a weired page with many ads and others. and it pops up ads from time to time when surfing the internet. in the favorites, it automatically adds many junk page bookmarks.
i am using windows-xp.
questions:
1). is it virus infected?
2). how to get rid of the bookmarks in favorites folders.
3). how to clean the virus?
Thanks a lot!