Sounds like your hosts file got hijacked.
Look in c:\windows\system32\drivers\etc for a file named "hosts"
Open it with notepad and look for lines such as
nnn.nnn.nnn.nnn www.msn.com
where n's are numbers. Delete all lines OTHER THAN
127.0.0.1 localhost
Also, I would guess that you have some spyware. You might want to
download and install Ad-aware - it's free for non-commercial use.
ftp://ftp.download.com/pub/win95/utilities/aawsepersonal.exe