D'ye know, if my son ever asks me for some words of advice for life I sometimes think that one of the better ones would be:
"Don't mess with other people's computers" [particularly if they're working more or less OK]
However badly they're set up, no matter what a mess the filesystem is in, no matter how badly fragmented the drive is or how many orphaned clusters there are - LEAVE IT ALONE! They won't care about the speed increase because they've been quite happy with it going kachunck-kachunk for years, something subsequently won't work or they won't be able to find the humongous spreadsheet that has been living one their desktop since Windows 386....
Be pleasant, nod positively and change the topic to flying (you don't know anything about computers..)
[Also keep away from women with migraines, mothers, back problems and those who are too pretty]
And the answer to your question is - maybe! Depends on several things but probably won't make a big difference on a PII.