Here's the thing
...what one is capable of at this moment on a computer...may not be available in the next minute/day/month.
That's the basic premise behind backing up...the uncertainty principle of life, certainly life with computers
You can assume whatever you want, it's your system.
As for running the diagnostic...since you only have one hard drive, I would suggest it.
If you had two or more hard drives...your risk would be theoretically a bit lessened. But, as it is, if that hard drive fails...you have nothing. If your system partition (the one on which XP is located) becomes inaccessible, you currently have no easy means of accessing the other two partitions (even if they have no problems).
I suggest it, but it's your call.
<<Also I believe this to be a virus problem and not some hardware malfunction.>>
Malware can just as easily prevent use of a single hard drive...as a mechanical malfunction. Actually, it's probably statistically more likely that malware will adversely impact a system...than it is likely that a hard dive will fail, IMO.
<<...when i try to reinstall a fresh windows copy it goes and says: " no valid system partitions were found, setup is unable to continue."
bear in mind that the copy of windows i'm trying to install is located on my C drive (the actual setup file) since even my cd/dvd drive doesn't seem to burn stuff anymore.>>
Well...if you have XP open and you are trying to install XP on that partition...it cannot happen. XP must be installed from outside the partition you intend to install it on. I suggest you get a working optical drive.
FWIW: The "drive not accessible" error message typically indicates a hard drive/partition problem (when the partition/drive indicated is a hard drive), a search using Google and "drive not accessible" will confirm this.