Blank Menu Bar In Explorer And Blank Performance Tab

After a few days of work, the user cannot open new windows explorer or other application windows. Nothing happens if she tries to. If she closes one window, then she's able to open a new one. The "allowed" number of opened windows varies from time to time, sometimes being as low as 4, but sometimes around 10 to 12. The performance tab in the task manager is completely blank (see screenshot). Some Windows explorer menu bars sometimes are blank, and a windows logo appear instead of the menus (see screenshots).
Rebooting fixes the problem, but it comes back a few days later. The laptop is part of a domain, running generally the same applications as other desktops and laptops in the domain, running the same antivirus, etc. The virus definitions are up to date, and a full scan has been run. Even stranger is the fact that when it happens, Windows XP doesn't even seem to be unstable, nor does it crash in any way. Just seems as if there was a limit on the number of processes or handles or threads allowed to run.

Laptop: IBM Lenovo Thinkpad (model 63716NF)
CPU: Core 2 @ 1,8Ghz
RAM: 2 gb
OS: Windows XP sp3.
Swap file: initially set to 2046 mb and max is set at 4092 mb
Processor scheduling and Memory usage are both set to "Programs"

I have been searching the web for solutions but cannot find any forum post describing the same problem. Thank you for any help.

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