how much will it cost me to upgrade from XP Home to XP Pro.
You'll have to shop around to see what you can find. I just checked on
Amazon.com and they have prices ranging from $48 to $187.88.
If you buy this, make sure you buy the Upgrade version, which is less
expensive than the Full version. And the OEM version will not do an
upgrade.
Exactly why do you want to do this upgrade? Are you aware that XP
Professional and XP Home are exactly the same in all respects, except
that Professional has a few features (mostly related to networking and
security) missing from Home. For most (but not all) home users, even
those with a home network, these features aren't needed, would never
be used, and buying Professional instead of Home is a waste of money.
In general, unless you need to join a domain, you probably don't need
Professional.
will i have to do
a clean install.
No, not unless you buy an OEM version.
what would happen to all of my programs (MS Office 2007)
Doing an upgrade means that everything--programs, data, etc.-- stays
as it is. However bear in mind that an upgrade is a big step, and when
you do it, things *can* go wrong. So it's certainly prudent to be sure
you have a backup of any data you can't afford to lose.