Upgrading current operating system

I'm not new to Macs but I'm not very tech savvy so thought it best to post here anyway!

OK, brutal honesty here. I bought my Mac Book about 3/4 years ago, it's on 10.4.something
Recently I got an iPhone but frustratingly it won't run on my computer (even though it will run on my work PC which has an OS older than 4 years... grrr.... but that's another story).

Now one of the guys at work just got a new Mac and the OS disks in the box are 10.6.4.
So I am wondering if I can use these on my MacBook to upgrade the system?

But will it wipe the HD? Will all my files and programs be deleted?

I am also hesitant because I have a rather expensive Photoshop package that I paid A LOT of money for but lost the bloody disks and key when I moved house.

So I am reaching out to you very intelligent and enlightened people for your wisdom. Thanks!