First let me adress the problem before offering you the answer to your request....
The operating system should not be the cause of the problems you are having.
You did not list the spec's of the computer but if it used to run faster with no problems then this "slow down" came on gradualy...
I would suspect that there is either spyware or a virus running possably both.
You should try to figure out if this is the problem first because a "downgrade"will not solve the basic problem even if it seems to at first.
I have repaired several machines that the complaint was very similar to what you have stated and within an hour I removed over 500-1000 pieces of spyware that were running and slowing the system down.
You can do a very simple test/fix for spyware with two FREE programs that can be downloaded
HERE.
Download and install Ad-Aware se Personal and Spybot Search and Destroy.
These two spyware killers are the most commonly used programs to kill spyware and what one misses the other usualy catches.
read the info/help files to insure proper operation and update before useing and you may find that the computer will return to all it's former glory without an OS change.
This is what I would do first if you were to drop it off for service at the shop.And I would highly recomend that these two programs be installed no matter what your final solution is.They are two of the most important pieces of software you can use in todays world.Spyware is almost wosre than a virus.
Second if you do not have an anti-virus program.....GET ONE!!!
almost all of the retail ones are acceptable and all of them are better than nothing.
If however you are a cheapskate like me the best free one can be found
HERE .
Use the link to the free download and use it.
The frequent use of the above solutions are critical in everday computing NO MATTER WHAT OPERATING SYSTEM IS USED!!!
But if after trying that and you wish to boot to DOS it is very simple ...insert a win98 boot disk into the floppy drive and start/restart the system.
thats it like I said very simple.....
If you do not have a win98 bootdisk you can get one
HERE.
click on the link marked Windows 98 SE OEM | Alt1 | Alt2
(I use alt1 all the time)and save.Copy it to a floppy disk an there ya go....simple as can be.
But please try the spyware fix first and let us know how it worked out....I think you may be pleasantly surprised.....