Thanks for all of the suggestions.
I have a Q6600 running at 3.1GHz, 4 gig of ram, 32 bit XP so only 3.1 gig available.
Running Rosetta on the cpu and Seti on the gpu.
Rosetta can be memory-hungry, and is a likely cause for your performance-problems.
SIMAP is a good, low-memory alternative, but unfortunately has normally only work start of month.
World Community Grid has many health-related projects, some of them with minimal memory-requirements, so should be a good project to run alongside Rosetta, to decrease memory-usage but still do useful work.
BOINC also includes many options to suspend then user active, with (or without) removing applications from memory, if removed from memoryit should give you back a responsive computer fast (but you'll lose everything since last checkpoint). Also, it's possible specify specific programs that leads BOINC to pause crunching.
As for BOINC-versions, the current v6.12.3x-versions does have a memory-leak, AFAIK it's fixed in the v6.12.41-clients but strangely these isn't listed on the download-page. For the 32-bit windows-version, you can download
http://boinc.berkeley.edu/dl/boinc_6.12.41_windows_intelx86.exe
I've not tested the version myself, so use at your own risk...
Another possibility is to downgrade to v6.10.xx, it's possible these versions doesn't have the memory-leak.
Oh, and while the download-page does list v6.13.xx, atleast Rosetta@home isn't currently working with this version, and all your Rosetta-work will be invalid. Also, downgrading from v6.13.xx isn't supported, among other things you'll lose all work on client.