Diagnosing horrible performance on Server 2003 Installation

[Metre Meter]

I'm running a medium-sized website hosted on a virtual server running
Windows Server 2003 Web Edition.

In the past few months, performance *really* gone downhill. At first I
blamed this on certain embedded ASP code (not all affected pages used
it, but I suspected it could still be hurting them if it was inflating
memory usage). But performance is still horrible after a fresh reboot
when memory usage is low (it typically increases until levelling out
at about 270-330MB). And turning ASP processing off on those pages
doesn't help much either.

Where should I start when trying to determine what the problem is?

Is it possible that our host has started trying to squeeze too many
virtual servers onto one machine? Are we to blame? (The size of the
website has grown steadily and significantly, but traffic hasn't). Is
circa 300MB reasonable usage for w3wp? Is the MS Malicious Software
Tool sufficient for standard web security use?

Would appreciate any feedback. Thanks!

- MM

[Metre Meter]

Hyperthreading? As far as I'm aware, no. The processor is apparently
an AMD Opteron 270 (2GHz); I assume this is the processor on the
"real" machine running the virtual machine.

-MM

[Metre Meter]

Service Pack 2? No; having checked, it's running Service Pack 1; would
this make any difference?

To be honest, I think it's mostly fixed. There were a couple of issues
I found (after spending quite a while on it). The first was that a
sidebar menu that was dynamically generated (but doesn't change *that*
often) was being reloaded on every page- we fixed this by setting the
"EXPIRES" HTTP header setting.

The second was that a Perl-based redirect page that a large proportion
of links on the site go through was taking the opportunity to clean up
old files *every* time it was run. It wasn't needed anything like that
often, so we put a rand() in that reduced it to approx. 1 in 40 times.
(The slowdown was due to it reading the directory listing, which- due
to us having tens of thousands of files in that directory- *way* more
than the system was probably meant for- created a noticeable delay).

The feedback was still appreciated though. Is there a systematic
method that IIS users normally go through to diagnose this sort of
thing, or is it really a case-by-case diagnosis?

- MM