Can't get XCache to work on one domain.

[Adam Dresch]

I know this isn't a Plesk specific problem, but it's something to do with IIS I think

I'm running Windows Server 2008 (Standard)
With Plesk 9.2
I've installed the Xcache extension and it works fine on two domains, but not another
I have two VBulletin forums, one forum works with it, the other doesnt
The one which doesn't, throws out about 2 pages of the following

Warning: xcache_isset() [function.xcache-isset]: xcache.var_size is either 0 or too small to enable var data caching in [path]\

My XCache settings are

xcache.var_count 4 4
xcache.var_gc_interval 300 300
xcache.var_maxttl 0 0
xcache.var_size 128M 128M
xcache.var_slots 8K 8K
xcache.var_ttl 3600 3600

So clearly there's enough Var_size, not to mention the other sites work fine with it
I've tried increasing the size, but still no go
Someone on the VB forums had a similiar problem and he said it was because his site was set to use a "dedicated pool"
I checked the site's settings and the Use Dedicated pool is UNticked
I tried ticking, saving, unticking, resaving, but it still doesn't work

Any suggestions? heh

Also, is there anyway to update the versions of Mysql and PHP on a plesk server? they seem rather old by todays standards.