Before you do the warmboot (restart Winders), take a look at the available resources to see what it says.
Do this by right-clicking on the "My Computer" Icon, then select the "Properties" part at the bottom of the drop-down list. After selecting this, click on the "Performance" tab - the one to the extreme right side, and see what the available resources are - in percentage.
Should be the same / similar for XP (I'll check this on my Wife's machine).
Another helpful thing to have running would be the System Resources monitor(s). These can show when resources are dropping and what resources are being used often / frequently.
Might need to allocate more memory space to your Browser's Cache (disk and DRAM), physically purge the Caches (empty them), or do some patch updates.
With 512 MB of DRAM and an endless level of Disk Cache (Virtural Memory), you should not run low on memory space until you are multitasking quite a lot of stuff, printing to several printers and moving files to other W/S on a LAN - all at the same time.
Speed will suffer drastically when DRAM is limited and Disk Cache is "abused". You will also notice excessive Disk accessing during this time.
Maybe the message is "Not Real" - like IE thinks there is less than 10% available resources during this transition period?
I haven't seen one of those for a long time!
Used to get them when running long sessions of AutoCAD, Word, Excel, Acrobat, Photoshop + Scanning stuff (all within the same sessions), without a "Refreshing Warmboot" for over 10 hours.
Prior "Memory Overfloweth" error messages came in the DOS only days of yore
Back in 1993, used to get that transient "DOS Memory Overflow" error message when trying to cram Real Mode Applications into ~640K + maybe a little bit of UMB!
(Tweeks for this were - of course - the multiboot options!).