New Build Problems with Sound and Multi Tasking

Please help :?:

I have just built a new system with the following:
>Intel Core 2 Duo E6600 Conroe 2.40GHz (1066FSI) CPU
>ASUS P5B (socket 775) PCI Express DDR2 Mobo
>GeIL 2GB (2 x1GB) PC6400C4 800MHz Ultra Low Latency DDR2 RAM
>Tagan TG500-U25 500W ATX 2.0 Dual Engine PSU
>WD Caviar SE16 320GB 3200AAKS SATA-II 16M HDD
>Asus GeForce EN7600GS Silent HTD 256Mb DDR2 Graphics Card
>Windows XP Pro 32-Bit (Genuine)OS

All is well other than when I play any sound files, they start to stutter or jump (even MP3 files on my desktop). The 'hour glass timer' also appears on a regular basis, especially if I am downloading a file and trying to open a file such as word or excel at the same time; the file opening stalls for a few seconds before opening. Can anyone point me in the right direction please? I have the latest Mobo BIOS and sound drivers etc and the RAM has been fully tested. CPU runs at 47c at idle, which the supplier tells me is fine?
I am not a gamer or pc whiz of any kind, but I do use multiple applications for web site design and photo projects etc.

Any help would be very much appreciated