Virus scan, system restore, chkdsk, defrag, memtest . . .these are the kinds of tests and scans that come to mind off top of my head
pretty much any test or scan can easily be run by a home user although getting rid of a nasty virus or spyware can get tricky and sometimes it is easier to just reinstall Windows - it would help if you tell us what symptoms you are having
if you are expecting large speed gains from running defrag then don't, that might help a bit but probably not a lot
if you want to run a FULL manual defrag yourself
http://www.howtohaven.com/system/vistadefragmentation.shtml
use the -v -w option
here is how to run chkdsk (hard drive test)
http://www.vistax64.com/tutorials/67612-check-disk-chkdsk.html
you can get a free and highly regraded malware scan at malwarbytes, well known for it's ability to remove nasty virii
http://www.malwarebytes.org/
try those things first before you send it in
if the slowness started suddenly then check for any recent hardware or software additions you have made and one by one back out of them, or else try 'system restore'
http://www.bleepingcomputer.com/tutorials/tutorial143.html
if the tech wants to spend any time on a 'registry scan and clean' then forget about it, won't help at all in all probability