The suggestion just made to manually delete things is a good one.
If you feel like you need to install a new operating system, then there should be an archive and install option that preserves user settings and files. (Be sure to have a good full clone or backup before doing this, just in case something goes wrong.) That archive and install option does put a new operating system on, but it also preserves most settings and programs. On the other hand, it might preserve the things that you feel are slowing things down.
You might also explore using Migration Assistant or Setup Assistant with a full Time Machine backup or with a full external disk clone of your old system. I believe there are options to move over some programs/files from the old backup to a new fresh installation but not bring others over. My experience in doing this with system 10.4, 10.5, and 10.6 was that all the user authorization codes for MS-Office and Adobe CS software were indeed migrated over. No reinstallations were needed. I can't promise your experience would be the same, but that's what we experienced here.
If you want to be REALLY cautious, I would do a fresh new OS install on an external firewire drive, leaving your current system alone. Then boot off the external and run Setup Assistant (it works a bit better than Migration Assistant) to migrate things you select from the existing internal drive/install to the brand new fresh external install. Then boot again from your external drive, delete from it what you don't want, and if everything is looking fine, then you can erase your internal drive and clone the external system back onto the internal. SuperDuper and Carbon Copy Cloner can be used in this way and their web sites give instructions about it. I think you will also find info from SuperDuper and CCC on their web sites and in their forums about how well they preserve user activation codes for software packages. I think both of them do a good job with this.