Unarchiving a big archive should not be a problem, you are just seeing symptoms of something else.
Is your Linux partition near its capacity? Like 95% full or more? If that's the case, then many filesystems (including ext3, ext4 and reiserfs) will get a lot slower.
Is the disk I/O speed otherwise OK in Linux? Will the programs start in reasonable time, does browsing around the directories with a file manager feel snappy or slow? Have you tried some benchmarking program, such as bonnie++ or ionice?
Does the unarchiver allocate all the available memory? See top while trying to unarchive that package.
Sometimes the default cfq disk I/O scheduler can lead to odd problems during the long-running disk activity bursts. You might try deadline or anticipatory schedulers instead. For example deadline tends to be quite good with database workloads, so it might work in this case, too.
echo deadline >/sys/block/sda/queue/scheduler
echo anticipatory >/sys/block/sda/queue/scheduler
Replace sda with the name of your hard disk.
This will be a temporary change and normally I would not recommend changing the cfq in desktop use, but if you are having other I/O problems than this unarchiving thing, it could be worth a shot. Unless all this is just because unrar ate up all the RAM... :-)