You can have different iTunes accounts on the same computer, but they should be in different Windows accounts (usernames). So there's no need to buy a separate computer for your daughter, she can do it in her own Windows account.
Of course, all members of the family logon into their own account already? If not, that's the first step to do, creating those users.
Can you have more than 1 apple device (itunes) account on the same computer. My son has an iPhone 4 and my daughter wants an Ipod touch but my son says that you can not put different itunes accounts on the smae computer. Is this true?